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macrumors newbie
Original poster
May 3, 2017
I'm interested in buying a brand new and sealed Late 2015 27" iMac with 2TB Fusion Drive from somebody.

This model will have 128GB SSD directly mounted I believe, plus the HDD.

My question is, can I use an external SSD via cable (USB or Thunderbolt) rather than the slow internal HDD?

So I would have 128GB SSD on board and the external SSD to speed it up?

Does this method work well?


macrumors 68020
Oct 19, 2007
Blue Mountains NSW Australia
For mine when I had a 2013 with 3TB Fusion, noticed no drop in speed at all. The operating system is installed on the 128GB and photos etc go onto the platter section as fast as the Flash Storage asd 24GB of Flash bis reserved for writing tom th platter drive.

If you used a large amount of the 2TB platter drive, then I think you would indeed see a slow down.

Having said that this time I went with 1TB Flash and itris faster than the Fusion, but one would expect that
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T'hain Esh Kelch

macrumors 603
Aug 5, 2001
The setup you propose is made for disaster, I would not recommend it. Accidently unplug the external drive, and boom, you lost a hell of a lot of data

Chances are you won't gain much of a speed difference anyway.


macrumors 6502a
Mar 8, 2017
Rio de Janeiro, Brazil
Actually, it will work fine.

You can move your entire home folder for an external SSD to speed up things. I'm using an external SSD from Sandisk, with 400 reading/wringing times.

It's not perfect as Internal SSDs from apple but it's much faster than Fusion Drive.

The Finder does not allow you to disconnect the external drive, as it knows it handle the whole thing. But if you accidentally have unplugged the SSD, Finder will freeze.

I've tested it a few times.

If you force quit it, it will ask for reboot. If you reconnect the SSD, it will unfreeze as nothing has happened.

It's a clever move!


macrumors Penryn
Feb 20, 2009
If you want the FASTEST SPEEDS POSSIBLE, that will ALWAYS remain that way, you need to do this:

1. back up the existing fusion drive to an external (for the moment)
2. "split" the internal fusion drive into a standalone 128gb SSD/flash "booter" and a 2tb HDD "general storage" -- TWO drives.
3. Put the OS, apps and accounts (excluding large libraries) onto the SSD/flash. Keep the SSD "lean and clean".
4. Put large libraries of stuff that isn't accessed frequently (photos, movies, etc.) onto the HDD.
5. Boot and run that way, managing the two drives as works best for you.

Any drive you "fuse" to Apple's [very fast] internal SSD/flash is going to "slow it down", and that includes external SSDs.
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