I've just made a Fusion Drive on my MBP 2012 and restored data from the Time Machine. Unfortunately, right now I read information that the best place for SSD is the main bay of HDD not the optical bay. So I want to replace SSD to the main bay and HDD to the optical bay.

My question is: it's possible without reinstalling the system?

I'm asking because I not fully understand how Fusion Drive works. When I make coreStorage and I put the names of disks (For instance, /dev/disk0 and /dev/disk1) how it determines where is the SSD and where is the HDD? By manufacture information? So it means that I can replace SSD and HDD vice-versa and it's gonna be fine or coreStorage stores information about each drive how it was determine: first - SSD - /dev/disk0, second - HDD - /dev/disk1?

  • 2
    Try it and if it doesn't work you can always switch them back Commented May 22, 2016 at 9:10
  • How about some links to where read the information. If everything is working OK, then the only reason I can think of for switching would be if you are not getting the full 6 Gb/sec transfer rate. So what is the transfer rate to/from your SSD? Do you know how to determine this? Commented May 22, 2016 at 9:32

1 Answer 1


I did the trick by Trent Fitzpatrick's recommendation. Everything works. Speed doesn't changed. Probably it's because I've just made a Fusion Drive and there need some time, probably it doesn't affect at all. But Write 120mb/s and Read 380 mb/s really slow for Fusion Drive I guess.

I'll wait some time and if speed doesn't will change I guess I'll replace SSD and HDD again because HDD tray has some shock protection.

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