External drive as boot drive -- 2020 iMac

A friend who is a heavy Adobe Lightroom user has a 2020 27 inch 5K iMac with a 2TB internal drive which is getting full. They are wondering about switching to an external 4GB or larger boot drive (with a second similar drive for backups).

Will an external drive achieve the same performance as an internal drive? Would RAID offer any real advantage at this point?

Would they be better off just moving their 1TB or more of photos to an external drive and continuing to boot from the internal?

Other recommended strategies?

They are currently on Big Sur 11.6.8


Assuming you don’t want to delete documents and you don’t want to crack open that iMac, you’re looking at external drives. And if that 2TB internal drive is a HDD, they should for the external just get an inexpensive SATA-based SSD interfaced through USB3. That won’t cost much (~$420/210 for 4/2TB with case and cable, even less if you choose a QVO over the better EVO) and it will blow the internal HDD out of the water in terms of performance. (it will be bootable too)

Best performance would come from a PCIe-based flash interfaced through TB in an actively cooled case, but that’s big time $$$ and likely overkill considering this is about extending the legs on a 2020 Intel Mac.


Lightroom is optimized for working well with external drives for photos and boot drive with library and cache files. If the internal drive is an SSD, will you still advocate moving the system over to an external drive?

Yes, her internal 2TB boot drive is an SSD.

If the internal is already flash, maintaining performance will be difficult without spending real money if you want to go fully external (again assuming you don’t want to crack open that iMac). A reasonable path forward instead would probably be to split up the partition. Keep the internal SSD as the boot partition with the system and apps. But move (some) media over to an external SSD. The less expensive variant (SATA interfaced through USB3) should be just fine for that purpose.


This is exactly my way of doing it. I keep the photos that I have not had time to edit on the fast internal disk. When I am finished editing, I use the tools in Lightroom to export photos and library as a unit. The photos I move to slow big cheap disks. The exported library I import into my aptly called OneBig library. Then I use Lightrooms tools to upgrade the OneBig library with the new path to the photos.

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Sounds like an external SSD for the photos is the best answer.

One question - is there a significant speed advantage using something like the Samsung 870 EVO and USB C 3.1 gen 2 enclosure, vs using a packaged drive USB 3.2 gen 2 drive like the Samsung T7 or LaCie portable SSD?


Just to clarify the slow big «cheap» disks in my case is several
WD Desktop Black 3.5" in external enclosures.

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I personally always buy separate.

  1. I save money
  2. I know exactly what I’m getting (this is where performance comes in)
  3. If either ends up having an issue, I can replace it and keep on using the other

But that said, there’s nothing fundamentally wrong with buying packaged. If you make sure to buy something decent performance should be on par (but at times manufacturers mix and match, hence #2). At worst you’ll be spending a bit more.

Regardless what you end up choosing, buy from a joint with a good return policy. If something ends up not being up to snuff, you want to be able to just send it back for a full return with no BS.

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She is spooked by build your own and seems to want to go for capacity, and is heading toward external Thunderbolt spinning disks, 6TB or 8TB, probably from G-Tech, maybe RAID. I can’t imagine why she expects her photo storage needs to grow so dramatically. Thanks for all your advice!

At least she also has a 8TB USB drive for a backup and also uses Carbonite cloud backup.

I’m in a similar position. My photos now occupy rather more than 1.5TB. I organise images into one folder (with lots of subfolders) per year. I keep those from the current year, previous year and, depending on space, the year before that on my iMac Pro’s internal drive and the rest on an external 2TB SSD (from Crucial, as it happens, although I don’t suggest it’s particularly important).

I find that setup works very well. I don’t often edit photos taken more than 18 months or so ago, so speed of the external drive isn’t terribly important for me and a fairly inexpensive SSD was fine. If your friend has similar needs, a spinning rust external drive may well be perfectly adequate.

One point: when it comes to it, the Finder does a far quicker job of moving a year’s photos to the external drive than LR, and it’s then a moment’s work to point LR at the new location.

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“Build your own” is a mighty big word for what is essentially the equivalent in terms of technical difficulty as emptying your vacuum cleaner. If capacity is paramount, you’re likely looking at spinning disks unless you want to spend real $$$. Of course if you want to save money and you are going for HDDs instead of SSD, the choice of TB over USB3 strikes me as a bit odd TBH.


I agree. My backups are to 4TB Toshiba N300 HDDs in external enclosures.

HDDs are great for cases where you don’t need high performance. Like backups, or documents you don’t access very often, or media files intended for playback.

Most external drives sold are actually internal drives pre-installed in an external enclosure. Depending on the specifics of what you’ve actually got, the DIY approach may perform better or worse than a pre-built one. If this is important, then you should research the products to find out what you’re actually getting.

In general, SSDs come in three forms: a 2.5" SATA device (designed to be used as an HDD replacement), an M.2 SATA device (a bare circuit board with a SATA interface) or an M.2 NVMe device (same size as the SATA version, but uses a much faster PCIe bus).

(There are also some proprietary formats, like the SSDs Apple uses for Macs that have removable storage. These sometimes look very similar to M.2 devices, but their connectors are different and incompatible. Companies like OWC make aftermarket SSDs with these interfaces.)

You can get external enclosures for any of the above, with different performance numbers. USB 3 is available with 5 and 10 Gbps interfaces. Or you can get Thunderbolt which has a theoretical top speed of 40 Gbps.

Important: SATA’s top speed is 6Gbit/s. Which means a SATA SSD will be faster than a 5Gbps USB interface, but its maximum speed is significantly slower than a 10Gbps USB or a Thunderbolt interface. So if you decide to get a SATA SSD, you should save your money and use a USB enclosure - don’t bother with a more expensive Thunderbolt enclosure unless you also get an NVMe SSD (which can take advantage of the faster interface).

But she really shouldn’t be spooked about building your own. It’s quick and easy.

HDDs are much slower than SSDs. There is no point to a Thunderbolt enclosure for an HDD unless you’re going to get a RAID enclosure with a lot of drives, but if you need that much storage, then you probably should start looking at a NAS or other kind of file server.

For the capacity you’re describing (6 or 8TB), use a USB 3 enclosure and save a bit of money.

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It really is very easy to “build your own” with the OWC Envoy enclosure.

I went that route with a 2017 iMac (18,3), and it works great. It’s my boot drive right now. I got a medium-high end (3,500/3,100 MBps read/write) 2TB Mushkin NVMe drive from Newegg last summer, put it all together no problem.

You just format the drive, boot from your recovery partition and install MacOS, then restore from Time Machine.

It’s a bit of a time consuming process, but my Mac boots and runs faster. :grinning: :+1:

Thanks all. I am certainly well aware of the ease of building your own, in my business years ago we built thousands of drives. Including the huge 2GB(?) Micropolis drives used by video folks. But that doesn’t mean my acquaintance was comfortable with doing it herself (I offered to do for her). Anyway she ended up buying a TB packaged USB-C external SSD, we’ll see how she does.

Meanwhile, it looks like the Thunderbolt SSDs are entering a lower priced arena, although it would have been possible to make your own near this price recently.