What can boot a 16" M2 MacBook Pro?

I’ve been struggling to get a WD Elements (2 TB USB 3.0) drive to boot an Apple Silicon Mac, running Ventura 13.5.2. The drive was cloned from the MBP by SuperDuper! via Apple asr. The MBP recognizes the drive as a valid Startup drive, but always fails early on with the same kernel panic: Library /usr/lib/libSystem.B.dylib not loaded. I think this means it can’t read the dylib cache.

For what it’s worth, I’ve tried:

  • First Aid (passed)
  • Safe mode (no effect)
  • Re-cloned
  • Different USB-C to USB 3 (type A) adapter
  • Different Thunderbolt port
  • Different USB 3 cable (didn’t even see the drive!)
  • USB 2.0 cable

About the only thing I haven’t tried is changing the security policy, which isn’t supposed to be required.

And, doing the Ventura install to the drive instead of the asr clone. The Carbon Copy Cloner help pages say that this is the definitive test for bootability. But I don’t agree with that: I have an OWC Envoy Express that can boot my 2017 iMac if I clone to it, but it can not boot if I try to install or update macOS on it.

So, I’m thinking that either Ventura or the M2 Mac just doesn’t like the WD Elements drive, which I’ll admit, is probably a really bad choice for booting. It was a gift. I’m just trying to boot from it to encrypt it and keep it as an all-else-fails way to get the MBP started.

Which leads to my question: If I’m going to buy a 1 or 2 TB portable SSD drive, with the intent of being able to boot from said drive, I don’t want to buy something and then have it turn out that the MBP can’t boot from it either.

For example, I’ve seen comments that say that USB-C drives are more prone to boot issues than Thunderbolt. (Despite the problems with my Thunderbolt Envoy Express mentioned above).

What portable SSD drives are people recommending these days, with the ability to boot an M2 Mac?

I bought Sabrent TB enclosures to use with my M1 Mini. Can’t recall whether I installed Samsung or Crucial SSDs: maybe one of each, One is my boot drive. They are very compact, seem very solid, and require no tools at all. I’ve read that you may be able to boot from USB drives, but I have major doubts about the one you mentioned. What drive is in that enclosure?

Sabrent TB 3 enclosure

It is a WD drive in a WD enclosure. Probably was the result of an Amazon search for “cheapest 2 TB portable USB drive”.

I have a Sabrent enclosure for a HDD I use to backup a Windows computer. I’m not impressed; it uses a non-standard USB cable with type-A connectors on both ends!

For that purpose, I wouldn’t try to clone the existing drive. I would erase the external drive and do a clean installation of Ventura to it (see Howard Oakley’s tips), and then, if I wanted my existing accounts and settings on the external drive, I would use Migration Assistant to transfer those to the external drive.

Sorry for the rant to follow, but I’ve basically given up and decided to throw out almost everything I knew from decades of working with Macs before High Sierra or thereabouts. Starting with Mojave, enough things work differently, enough things change frequently, and, perhaps most importantly, enough things are so poorly documented that I no longer assume that traditional system administration procedures still apply. Even for relatively basic tasks, I’ve either stopped doing them, or I research them thoroughly before trying them if they might impact basic operations or data integrity.

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Booting Apple Silicon Macs from external media has always been a challenge. It’s my understanding that it may be very difficult (and may actually be impossible) to make a bootable clone of your internal volume. From what I’ve read, it may be better and easier to clean-install macOS to your external device, then use Migration Assistant to copy your content to that newly-installed device.

Here’s Howard Oakley’s list of Apple Silicon Mac articles: M1 & M2 Macs – The Eclectic Light Company. Scroll down about halfway for a large block of articles about external boot disks.

These days, the common advice is to not even bother with bootable clones of any Mac running Big Sur or later. The signed system volume just makes it awkward and difficult for the result to actually be bootable. Instead, make non-bootable backups (including clones and Time Machine). If your system dies, you can reinstall macOS (via the Recovery volume) and then use Migration Assistant to put back everything that was on your Data volume.

Interesting, but note that the boot process for Apple Silicon macs is completely different from that used on Intel Macs. So I don’t think those results are going to be meaningful.


While having a bootable backup for this machine isn’t that important (it isn’t my primary computer), I think bootable backups are one of the great features of Macs, if not fully appreciated. It would be a great “Do you use it?” poll.

The point to a bootable backup is that it reduces your downtime to minutes. If you’re going to have a full backup on external media, why not make it bootable?

That being said, I’m a belt and suspenders kind of guy. I have multiple backups, including:

  • Time Machine to an external SSD
  • 2nd Time Machine that I only connect once a month
  • Full clone to SSD, updated weekly
  • Another full clone, updated monthly
  • Two full copies of data, updated alternatively every 4 weeks and rotated offsite
  • Backup to cloud (Wasabi) using Arq, going back 5 years
  • Key files backed up again to Dropbox and synched to another machine
  • …which is also backed up with Time Machine, clones, Arq

Maybe I have a FOLD (Fear Of Losing Data).

The bootable SSD saved my bacon when I couldn’t get the iMac to update to Big Sur, but could update the SSD (*). (I needed to get the iMac’s internal SSD replaced). I swear that the external SSD was world’s faster than the iMac’s internal Fusion drive. I was sorry to move back to the internal, but at least I could start apply macOS updates again.

(*) but that was the last time I could, because starting with Big Sur, I could no longer apply updates to the SSD. OWC says it is a an Apple problem.

Don’t get me wrong here. I also love the concept of bootable backups. And when it was possible to make them and keep them up to date, I used them all the time.

But Apple has made it increasingly difficult - to the point where it is now approaching impossible - to do this on Apple Silicon Macs. So I can no longer recommend the process as a practical recovery solution.

And yes, this is a huge step backward - going back to the way I worked 20 years ago when I was backing up to tape instead of external hard drives.

But I think (not completely sure) that you can get the next best thing. If you make an independent macOS installation onto external media, then I think you should be able to clone your internal system’s data volume to your external system’s data volume as part of an incremental backup strategy. Your external clone won’t get macOS updates this way, so you’ll need to periodically boot from it and run Software Update to upgrade the System volume.

I think that will work, but I strongly recommend testing it to be sure. You don’t want to depend on my word for something as critical as your system.

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It sounds like the WD is probably a spinning HD, and if so, it’s just not suitable for use as a boot drive with an M1 or M2 (or for that matter, for use with any OS later than High Sierra due to the APFS requirement—High Sierra can at least still be run on HFS+).

Did you look at the link to the enclosure? That Sabrent enclosure uses TB cables. Nothing weird about them.

An HDD can boot a modern macOS installation, but as you pointed out, the performance will be so bad it should only be used in an emergency (e.g. if you have no other options remaining). This is partly due to the nature of APFS, but also due to a variety of other macOS design decisions, which have been making HDD booting less and less attractive, going all the way back to Mac OS X 10.9 (“Mavericks”), becoming successively worse with every release since then.

That having been said, I did make and test a bootable backup on my 2018 Mac mini when it was running macOS 10.15 (“Catalina”). It worked, but the UI was so sluggish, I didn’t let it run longer than the few minutes needed to shutdown the system and boot back to the internal SSD.

This was with a 7200 RPM HDD mounted in a USB3 enclosure. I shudder to think of how bad the performance would be using the latest macOS and a portable HDD (which tend to be 5400 RPM or slower and are poor performers even compared to other HDDs).

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A friend of mine is ready to throw her 2017 iMac away because it is now unusable. I had asked her to check with me before upgrading the OS, but she didn’t. When I took a look at it this summer, performance had become excruciatingly awful and the iMac was unusable. A look at the system specs (it took at least 5 minutes to get that info to appear) showed that her iMac had shipped with a 5400 rpm spinning drive. I am still stunned that Apple shipped a system in 2017 with such a POS drive in it. Maybe the drive is failing, but finding out would be painful. Trying to boot from an external would also be painful and time consuming.

To make matters worse, every time she contacted Apple about the problem, they told her to upgrade the OS—which was stunningly bad advice, if they bothered to consider the specs of that model.

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Yuck. Painful. And you’re absolutely right - Apple should not have ever sold a system with that configuration in 2017.

That having been said, the HDD is replaceable, although it is a bit difficult so you may want to enlist a third-party repair shop to do the job.

You should be able to install any 2.5" SATA SSD in its place. For example, a Crucial MX500 series or a Samsung 870 EVO series.

If you find that too challenging and you don’t want to pay a repair shop, the other alternative is to buy a Thunderbolt (or USB 3, if you’re on a budget) external SSD, install macOS on it, and use it as your boot device. After everything is fully migrated, you can (if you choose) wipe the internal 1TB HDD and use it for document storage.


There is a Mac repair shop not too far away from us. I used to go to Rossmann’s, where they always did an excellent job, but unfortunately, they left NYC, and in any case, they didn’t work on iMacs. Someone from Rossmann’s recommended this place near me, and I am going to suggest it to her. But first I will have to find out if they work on iMacs. I wouldn’t open that thing up myself for any amount of money.

Just getting that iMac to reboot to an external drive would be painfully slow. Glaciers zip right along in comparison.

A while back, I went so far as to prepare a test/repair Monterey SSD in a USB 3 enclosure, with a few repair apps added. But I’m not sure how much tolerance my friend will have for trying to fix the thing. Also not sure if she has a firmware password, or if she would know if she ever assigned one.

She bought it with plenty of RAM, but it didn’t occur to me to have her ask about the drive, because it never crossed my mind that Apple would ship such a pathetic configuration.

I’d second the suggestion to just have a professional swap that silly HDD with any decent SATA SSD. But it really comes down to the cost of that operation (labor). For comparison, even an external USB3-based solution that is no doubt less robust (your friend would be far from the first non-pro user to accidentally disconnect their external boot drive) will at 2TB run you at least $110 with a cheap case at least $80 with a cheap case. It’s questionable if it’s worth still putting that kind of money into a 2017 iMac or if that would be rather put towards a new Mac. Obviously, that depends on the specific use case and what other investments were put into that station.

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Since it’s not an iMac Pro, macOS Ventura is the end of the road for operating system support. 2017 iMacs (except for iMac Pros) will not support Sonoma. Ventura should get another two years of security updates, but Simon’s suggestion certainly is reasonable to consider.

Right, but that’s a lot of life. As you point out, Apple will likely provide OS updates for two years, and maybe an occasional update after that if there’s a very critical security problem.

And if you’re OK with not using Apple apps for Internet content, then you can probably take a few more years until your Internet facing apps (web browser, office suites used for downloaded content, etc.) drops support for Ventura.

As for performance, I think that 2017 iMac will work well if the HDD is replaced with an SSD, internal or external. If you install it internally, spring for a 4TB drive, just so you don’t have to worry about filling it for the remaining life of the computer.

I’d also max out its memory. If you’ve got 16GB, that’s probably enough to last the life of the computer, but if you are going to pay a shop to open it up, you may as well install new DIMMs while it’s open. @kat634e didn’t say which 2017 model we’re talking about, but you need to remove the display to access the RAM on a 21.5" model, so it pays to do that upgrade if a shop will be opening it to replace the HDD anyway.

If it’s a 27" model, ignore that last paragraph, Upgrading the RAM on a 27" iMac is trivially easy, so you can defer that upgrade until you actually need it.

Just remember, if that internal SSD should fail on an Apple Silicon Mac, you ain’t booting nuthin’. Unlike Intel Macs, all booting on an M1/M2 Macs needs components that are on that internal drive. It’s all to do with security.

That’s one reason why a cloned drive for Apple Silicon Macs isn’t as useful as it used to be.

I have a 2017 21.5" iMac, which I configured back in the day with a 1 TB Fusion Drive. The performance was terrible; slower than the iMac (with a 7200 rpm HDD) that it replaced. The most disappointing Mac purchase I’ve made. Shortly afterwards, I bought a Samsung T5 external SSD, migrated everything to it, and made it the boot drive. Relegated the internal Fusion Drive to a CCC backup drive. Huge improvement in the iMac’s performance.

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I believe she has this one or one very similar:

21.5" iMac specs

*This model has a Serial ATA (6 Gb/s) connector for a 2.5" hard drive. If configured with a “Fusion” Drive or an SSD at the initial time of system purchase, it also has a PCIe connector, but this connector is not present if the system only is configured with a hard drive.

At the time of purchase, Apple offered a 1 TB “Fusion Drive” (which combines a 24 GB SSD and a 1 TB hard drive) for an extra US$100, a 256 GB SSD for an extra US$200, or a 512 GB SSD for an extra US$400.

We haven’t discussed the iMac in quite a while. Her needs are very simple, so more than 16GB of RAM would be pointless.

This may be the first thing we try, if she even wants to bother. She was frustrated enough that she wanted to just throw it away at one point.

As @TheJohnB wrote, the easiest approach is an external (USB or TB) SSD. You can just buy it, hook it up, and install macOS to it. And then use that SSD as your boot device.

It might even outperform an internal SATA SSD, since SATA is limited to 6 Gbps. USB3 Superspeed may be 5 Gbps (slower) or 10 Gbps (faster), and Thunderbolt may go up to 40 Gbps. Of course, the SSD itself, macOS and your usage patterns will affect how close you come to the maximum throughput of any interface.

I only suggested a RAM upgrade as something to do if you’re going to replace the internal HDD with a SATA SSD. Not because you necessarily need it, but because RAM is relatively cheap and the labor to install it on a 21.5" iMac can be high. So if you’re going to be opening the case anyway, it’s worth considering.

But if you aren’t going to be opening the case (that is, using an external SSD), then there’s no point to a RAM upgrade if you don’t actually need it,