Creating a Fusion Drive


macrumors regular
Original poster
Nov 13, 2007
Victoria, BC
Hi, I have a 2013 27" HD based iMac (Mavericks installed) and a 120GB external Samsung 840Evo SSD in an USB 3 enclosure and I'm wondering if it's worth combining them into a fusion drive. At present the hard drive reads and writes around 175mb/s. Are there any downsides besides one of the drives being external/vulnerable to disconnect?

1) Does this void warranty
2) Does this limit the life of drives?
3) Do time machine backups continue to work from an existing backup?
4) Would a DIY Fusion Drive create problems for future OS X updates?

Thanks for any help!
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macrumors Core
Feb 20, 2009
My thoughts:

The SSD you have now will run faster "as a standalone SSD" than it will as being one-half of a "fused drive".

IF you don't mind having the two separate drives on your desktop, and
IF you don't mind managing them _as_ "separate drives",
My suggestion is to "let it be"...


macrumors 601
Oct 26, 2009
I wouldn't do this strictly from a reliability standpoint. If the external got disconnected it would potentially corrupt the entire drive pair. I'd rather keep the data separate instead of mingled!


macrumors regular
Original poster
Nov 13, 2007
Victoria, BC
Thanks for the input!
I was thinking that the 840 Evo would roughly triple speeds (?) and I work in many programs (photo editing software). I'd secure the cable to the self powered enclosure and perform regular TM backups.

Has anyone had problems doing this before on Mavericks? Is there not more benefit than doubt?
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macrumors 6502a
Apr 30, 2008
USA (Virginia)
Well, personally, I would try it. Have good backups, of course. Having those, you could always go back if you didn't like it for some reason. I'm elated with the homemade Fusion drive I set up on my older iMac running 10.8.5, but I'm using both internal drives. (I also have a very large SSD -- 512GB. Much larger than necessary and a long boring story about why.)

I think the "drive gets disconnected" worry is exaggerated. I've had external Firewire drives for years, and never an accidentally disconnected cable. YMMV, of course. But, even if that happens, wouldn't you have a backup anyway? Just reconnect and restore, I figure. For me, risking an hour or whatever restoring is not a big deal.


macrumors regular
Nov 23, 2005
I recently upgraded to a Late 2013 27" iMac, and was looking to do the same thing...create a Fusion drive combining the internal 1 TB HD with an older SSD I have in a USB 3.0 enclosure. After spending the entire weekend so far trying to get it to work, I've determined that it's not possible. The Mavericks installer seems to have issues with creating a complete OS X will seem to complete the installation, and will have the files residing on the Fusion drive, but it errors out saying that it couldn't create a recovery partition (even though one already exists). I even tried cloning a known-to-work Mavericks install from another partition to the Fusion drive...the iMac will recognize the drive in the startup disk prefs pane, but the system doesn't see it upon reboot. I'm thinking Apple has added some firmware code to this model to disable DIY Fusion drives (at least those that incorporate an external drive).

Anyone else with better luck?


macrumors 6502
Jan 10, 2014
Sydney, Australia
As you are not opening the machine, it will not void your warranty. While it is extremely minimal, it does do some behind the scenes sorting - writing files to each drive. It shouldn't matter either way.

As for the time machine backups, I am not 100% sure but I believe that it will delete the previous backup and start again. And, since Apple officially use fusion drives in their machines, it is highly unlikely that there would be an issue unless Apple stopped making fusion drives and removed the functionality from a future OS version.

Despite this, since the SSD is external. I would strongly suggest that you keep them separate. They could be serious issues if the USB drive gets disconnected. Oh, one more thing, why did you choose to get a USB 3 SSD drive? Thunderbolt is so much better for booting.


macrumors 68040
Aug 25, 2012
I found this ssd, crucial mx100 512 gb ssd for 199. I'm trying to do the same thing. Good deal?


macrumors 6502
May 15, 2011
I haven't tried creating a fusion drive with the SSD external, but I did pull out my SSD which was fused with my HD,hooked up using a spare USB3 adapter and managed to boot the system just fine.

You might try time machine restoring onto the fusion drive - this generally doesn't create the recovery partition. Or you could install to the HD (so there's a recovery partition), then fuse the HD's partition (diskxs2 probably) rather than the whole drive. That should leave the recovery partition intact, and it might figure out not to make a new one.

I've fused just partitions before, though generally so I can have half my SSD used for windows.
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