External SSD for Boot Drive: Questions on format etc


macrumors 6502a
Original poster
Aug 17, 2010
App Q&A testing by request.
Thanks to advice in another thread I'll be setting up an external SSD (via USB3) for a boot drive due the internal SSD portion of my fusion drive starting to fail.

I got the drive today (a 1tb Crucial) and I have some question about the best ways to proceed.

First, should I go with APFS or HFS+ ? I won't be using encrypted in either case. The iMac is High Sierra, and I imagine I'll eventually upgrade it to Mojave (though I only did HS a couple of months ago, so I won't be in a rush. The current drive is HFS+, as it's a fusion drive, and when I was given the choice, it said APFS doesn't support that.

Following from that question.. IF I go with APFS, can I still clone the current drive over to the new drive? That IS my current plan, but I really don't know if that would be feasible with different file systems.

Additional to that is, IF I go with APFS AND can clone, will my Time Machine backup...currently HFS+....work with the new drive?

ALSO, am I forgetting to consider something that someone with more experience might think of?


macrumors Core
Feb 20, 2009
Use HFS+.
It works.

I was the one who wrote the advice you quoted from another thread.
Don't over-think this whole process.
Just.... do it.

Erase the SSD to Mac OS extended with journaling enabled (GUID partition format).
Then... run CCC.

Once you see how easy creating a clone is, you won't be fooling with TM much more.
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