An External SSD Gave My iMac a New Lease on Life

Significant price difference at same capacity is often related to interface. SATA is cheaper, PCIe-based (NVMe, M.2, etc.) is more expensive.

If you’re putting the SSD in an inexpensive USB-C case or dock that won’t matter a whole lot. But if you are planning to use TB3/2 or USB3 Gen 2.2 because you need best performance, you will want an SSD interfaced through PCIe, not SATA.

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Dine…not that I’m aware of beyond it just not being updated. It was more of an ‘there was a reason they updated it and it might be a security related thing’ than any actual problem.

Out of the box, macOS will not even attempt TRIM on non-Apple hardware.

You can, however, run the trimforce command to let it work on third-party SSDs. (Don’t forget to reboot after running this command - the change won’t take effect until you do.) Whether this will work on USB devices or if it is reserved for other interfaces (e.g. SATA and Thunderbolt), I don’t know.

If the trimforce command is something you’re not comfortable using, there is a commercial product, Trim Enabler which does the same thing, but with a really friendly GUI (and several bundled diagnostic tools).

In theory, it is possible to run TRIM over USB if the enclosure supports UAS, since SCSI’s “UNMAP” command performs the same function. But even if your enclosure has support, your computer’s OS still needs to send the command.

But it should also be noted that some SSD manufacturers claim TRIM is unnecessary. For example OWC says this for the drives they sell. Some people disagree strongly with claims like this, but that’s beyond the scope of this discussion.

This isn’t too surprising. SMART requires support by the device as well as the operating system. If the T5 USB interface doesn’t provide support, then there’s not much software can do about it.

Out of the box, macOS will not even attempt TRIM on non-Apple hardware.

I have a Thunderbolt Envoy Express with WD NVMe blade, which SI shows as “TRIM support: yes” which is the same as it shows for the internal drive. I assume this is because the system treats Thunderbolt as indistinguishable from the internal. This is without any action by me.
I haven’t looked into TRIM for a while but I think trimforce and Trim Enabler don’t work for USB on Macs, but do on Windows if the spec allows. (UASP rings a bell).
Rightly or wrongly I have never been that concerned about TRIM since the early days of SSD because of what you say about ‘Garbage Collection’ being sufficient these days. Most of the very popular USB-C SSD drives like Samsung and Sandisk are out there without TRIM. When I used Trim Enabler long ago it was on internal SATA drives.

I have a 2013 iMac, & had the Fusion Drive swapped out for a 2 TB Samsung SSD a couple of years ago. Made so much difference, & the original SSD from the Fusion is still there as extra Storage (I have Win 7 on there for a few old games :grinning:)

At my office I have a 2017 21.5 inch iMac and stupidly thought I could expand the RAM but I cannot of course without taking it apart. However I needed to speed up the machine just to be useable as the fusion drive slowed right down with Mojave as you saw too. So I got a UASP class USB3 enclosure and put a Samsung EVO 850 512gb drive in there and since I did that in early 2019 it has worked great. I did the same thing with the 2014 mini at Church which has the 5400rpm drive in it and that helped too. Nice to see that others do the same too! My 2012 mini has 16gb and an SSD in it now after buying that off of eBay for $250 back in 2017.

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Hi all,

Did the same. Highly recommended. Boot time went from > 50 s to less than 25 s. More importantly, going through my 80 Gb photo library is fun again.
Finally, use a real TB3 enclosure and get the fastest SSD you can. It makes a difference!


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Reading this article was like deja vu, I went through almost the same process last fall. I had purchased a used 27" 2017 iMac (3.8 GHz quad-i5, 40GB, Radeon 580) that came with a 2TB fusion that the previous owner has split. Was my first time with a fusion drive, so didn’t even know they could be split into separate devices. Was easy enough to re-join them since I didn’t have any data on the machine.

Being a fairly high-end machine it was still very usable but not as responsive as my old MBA with an SSD I was replacing. I was thinking about how I wanted to back up that 2TB drive when I had a different idea. I decided to upgrade to an external SSD as my main drive, and use the internal Fusion drive as an cloned bootable backup. I had used a waterproof external (Silicon Power Armor A80)* for work in the past for my startup drive so I wouldn’t have to bike with a laptop, so this setup was familiar/comfortable for me.

Since this would probably be my startup drive on my main computer for the foreseeable future (easy to migrate to a new mac by just plugging in an external), I decided to go for the higher performance of a NVMe drive as well. I went with the WD Black SN750 1TB drive in a TEKQ Thunderbolt 3 enclosure. I also looked at SanDisk’s Extreme Pro drives, but the price was higher and performance was lower.

The enclosure does get warm, but not hot, and I have it resting in the Vesa monitor mount in the back of the iMac. CCC does a nightly clone to the internal fusion drive, making much better use of the slower 2TB drive.

I measured the drive speed and it was over 2000MB/s for both read and write. Note: if you are getting separate enclosures and NVMe drive, you need to check each of them for what speed they are rated for. See below for tech details.

There was a good discussion on MacRumors that discusses many aspects of this exact upgrade including more of the technical details of different options.

*I’ve used/still own 4-5 different non-SSD Silicon-Power Armor A80 and A85 drives including living/traveling throughout Asia with a lot of time on dive boats and never had a problem with any of them. Great drives.

Tech Deep(er) Dive:
When researching NVMe enclosures, there are a few things to be aware of. As mentioned in the article, they are PCI based, so just like USB, there are different versions/speeds and configurations possible that affect performance. The controller used for NVMe enclosures/drives makes a huge difference. Just because it’s thunderbolt 3, doesn’t mean it will be that fast. You need to also check the number of PCI lanes a controller uses, or at a minimum what speed the enclosure supports as well as the drive itself.

As an example, I was considering getting the OWC Express 4M2 enclosure, which is a 4 slot NVMe enclosure. I was thinking I’d start with one or two drives, could mirror them, and expand as needed in the future. But what took a lot of digging to find out, each slot in the enclosure only uses 1 PCI lane, limiting each to ~750MB/s. To get the full listed performance of 2800MB/s you’d have to fill all four slots and stripe a volume across all of them. Putting a fast 2500MB/s NVMe drive into that enclosure would be a waste, since each drive would be limited to 750MB/s, and if only using 2 drives, the speed would also be limited to ~1500MB/s. To be fair, that’s probably also why it was half the price of a single drive enclosure rated for 3000MB/s.

There was a review of the TEKQ and similar drive that I found useful. It’s also a good historical reference for how much prices have dropped.

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Thank you Glenn for this article. I bought a 21.5" iMac in May 2018 with a 1 TB rotating drive. I’ve owned Macs since probably 1990 and except for a Performa, this was the worse purchase ever. Starting Excel could take almost 2 minutes
I am at Mojave and followed your procedure to use the external OWC drive. The Excel file opens in 4 seconds
My only challenges are with Time Machine It seems I will have to erase and restore the TM drive(s) as they don’t recognize the new Boot drive

Again THANKS Hugely


I know this is slightly off topic, but where can I find info on the “worthwhileness” of swapping out an internal 1 TB spinning HD on a Mac-Mini Late 2012 (High Sierra 10.13.6) for a new 1 TB SSD kit that OWC sells. I’m not a power user, but I’d like applications to load faster and put off buying a new Mini for several years. Any pitfalls/uninteneded consequences (such as the BootROM issue discussed above) to consider?

Replacing a boot HDD with an SSD is really a no-brainer. The only question in your case is if it’s worth investing in a 9-year old Mac mini. To some it would be, SSDs aren’t expensive after all. To others, it would make more sense investing that money into a new Mac mini.

I have a 2012 Mac Mini i7 Quad Core and a few years ago added an external Toshiba SSD for a boot drive. I didn’t want to change the two internal spinning drives since I decided to use those for backups as well as boot drives for older systems which I have kept going all the way back to Mavericks just in case I ever needed them. I noticed faster boot times right off the bat and everything seemed more responsive. Since the SSD is a USB 3 device, it’s not going to be as fast as an internal SSD drive but for my needs, it works just fine. I’ve never had any BootROM issues to my knowledge. I mainly use my Mojave partition but also have one for Catalina which updated my computer to 421. at some point. Since SSD’s are not that expensive, it’s worth investigating. The only issue with a 2012 Mac Mini is you cannot upgrade to Big Sur unless you try one of the “hacks” out there which I would try if I had a spare computer but don’t want to brick my main computer trying to find out.

Great to hear your SSD switch was a successful! However, you may be able to use Time Machine adoption to shift the old backups to the new one. If your new Mac inherits your backup history - Apple Support and How to fix the wrong Time Machine adoption for a drive swap

I’ve got a 2013 iMac that I’m thinking of doing this with. I’ve got a spare M.2 SSH drive so just need an ext. case for it the right cables.

That is where I’m stuck. I think my machine only supports USB 1 and Thunderbolt 1, so looks like Thunderbolt is the better choice. But the only option seems to be the Apple Thunderbolt 3 to 1 adapter and then a thunderbolt 1 cable. This is looking quite pricey for me. Did I miss another option higher up in this thread?

EDIT: seems I was looking at wrong spec sheet. 2013 iMac supports USB 3.0. I’ll try that.

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For a PCIe-based SSD even TB1 will produce clearly better results than USB3, especially your old iMac’s 5 Gbps flavor of it. That said, the TB solution will be far more expensive.

The main calculus surrounding whether a Thunderbolt 3 SSD and an adapter for Thunderbolt 2 will be worthwhile is if you’d have significant use for the external SSD after upgrading to a new Mac eventually. I’ve been very happy with SSDs for Time Machine and duplicates so I don’t have to listen to hard drives constantly spinning up and down.

One non obvious problem is that creating a clone with carbon copy cloner creates a Mac volume that can only be upgraded with software update from preferences. If you run BackBlaze and then boot from a clone with an earlier version of Big Sur Back Blaze gets very unhappy. Probably it would be better to leave Backblaze off the clone and then install it when running the clone and inherit the backup from the one originally used on that machine - as if it were a new computer. Not realizing that cast me a couple hours of time.
So BackBlaze users need to think out how they want to manage their backups when booting from a clone.


I had this problem while trying to create a test clone for a test Big Sur update. Backblaze recognized that it was a different disk, locked the backup, and when I booted back to the original drive I reinstalled BB and inherited the old backup, but you cannot really just turn it off and on. The support staff for BackBlaze were very helpful and sorted everything out for me.

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EDIT: seems I was looking at wrong spec sheet. 2013 iMac supports USB 3.0. I’ll try that.

I ended up getting a $10 USB 3.0 M.2 SSD case from Amazon (already had a 256GB m.2 drive that I took out of another machine) and it arrived a day later. Formatted it as APFS and did a fresh install of Catalina to it. Seems snappier than the standard spinning drive in the iMac (not a fusion drive). I only use this machine in my garage for Zwift so happy with that $10 outlay.

Thanks again for this article.

I was excited about moving my photo library to one of these. But the price for the 2TB is a frightening $500.

There are cheaper 2TB NVMe SSDs out there, but their transfer rates are much lower:

What makes one NVMe SSD faster than another? For spinners, you could estimate the transfer rate based on the rotational spin and seek time. Is the difference here purely about how their firmware is striping the flash storage?