Replacing internal 3TB HHD from Fusion Drive with a SSD and using 3TB HHD as external

Discussion in 'iMac' started by lazlo kovac, Jun 16, 2013.

  1. lazlo kovac, Jun 16, 2013
    Last edited: Jun 16, 2013

    lazlo kovac macrumors newbie

    Jun 16, 2013
    Hi guys!

    I own a late 2012 iMac 27" with 3TB Fusion Drive. I'm into video editing (amateur at the moment but I'll turn professional in a few months), and I'll need higher writing and reading speeds, but without sacrificing storage. According to Blackmagic Test, my iMac currently has writing speed of 280 MB/s and reading speed of 350 MB/s.

    I've been reading quite positive reviews about the Samsung SSD 840 Pro (with speeds of both writing and reading above 500 MB/s), and I'm considering the following:

    1. Replacing the internal 3TB HHD (just the HHD, not the unremovable Apple blade SSD) with a 256GB Samsung SSD 840 Pro.

    2. Using the 3TB HHD as a external drive enclosed in the Seagate GoFlex Thunderbolt Adapter.

    3. Fusing the three drives (I don't even know if this is possible, but according to some articles I read this might be feasible).

    So what do you guys think? Have any of you done something similar? Any suggestions/comments/feedback on this?

    Thanks in advance!
  2. cmanderson, Jun 16, 2013
    Last edited: Jun 16, 2013

    cmanderson macrumors regular


    May 20, 2013
    You may be able to create a Core Storage volume with the three drives, but Fusion Drive technology was not designed to work with a) external drives and b) more than two drives in a Core Storage volume group.

    With Core Storage (not Fusion specific), you're essentially concatenating the drives (or vdevs, if you choose to use partitions instead of whole drives.) This is limiting in that you're turning control over to the OS as far as what gets written where.

    Assuming you add the Samsung SSD first, then the Apple Flash before the 3 TB external drive, the first 384 GB or so would likely be written internally before writing to the external disk. This is actually what a lot of the first "DIY" Fusion Drive users ended up with, nothing more than concatenation and not a fully functional Fusion Drive proper.

    As you probably already know, Fusion operates at the block level and will dynamically move contents of your slower HDD to Flash memory, with a 4GB "buffer" for writes to the volume. That's roughly how it works, I don't feel like regurgitating the same information others have gone into great detail about on MR and elsewhere. At any rate, many of the initial DIY'ers weren't experiencing the block level migration (storage tiering) unless they had a Mac Mini that shipped with the "special" version of Disk Utility that wasn't widely available until 10.8.2 or 10.8.3.

    Additionally, if you were to lose power to the external disk, you'd risk corrupting the entire volume. At minimum you'd lose access to the OS immediately. Therefore, I feel this is not a good idea.

    If you were me you'd consider the external drive for a new or additional Time Machine backup (can't have enough backups), to store your media collection, etc.

    If you want to use Core storage with external drives, use it only with external drives (preferably in the same cage, sharing a power source.)

    I haven't tried it, so I can't tell you what type of issues you'll run into, other than to say it's going to be interesting what OS X might show you in Disk Utility when it sees the volume and mix of Flash, SSD and 3TB external drive.

    Quite possibly, you'll see a message about a broken Fusion Drive, but I doubt if you do see that message the offer to repair will work. I believe Apple has tweaked the code to recognize external drives and refuse to "Fuse" them into an FD volume.

    As far as Fusion Drive for internal drives, no reason I can see that it wouldn't work with the Apple Flash memory and Samsung SSD. However, it would be another interesting scenario since you'd have less space on the slower device. So more would end up on the faster Samsung 840 Pro, with only up to 120 or 128 GB remaining being stored on the Apple Flash drive. If you never used more than the capacity of the Samsung SSD, you'd never have any data written to the Apple Flash drive (but that's not a bad problem to have.)

    Since your intent is to "go pro" in a few months, and video editing typically requires a lot of fast disk (HD projects can just chew it up for a feature length production), I'd suggest looking at dedicated external storage. I'm a fan of the LaCie 5big in RAID-0. You would be able to work with multiple 2K streams simultaneously on a unit such as that, but it's expensive and crazy expensive for the 20TB model.
  3. Raima macrumors 6502

    Jan 21, 2010
    If your video's aren't that large, why not just use the Seagate GoFlex Thunderbolt adapter and the 256GB Samsung 840 Pro externally?

    If you can afford a larger SSD, go for that.

    That way you'll be able to do all your work on the drive externally without voiding warranty or damaging your machine in the upgrade process.

    Once you've completed the work, move it off onto slower larger storage solutions.
  4. Bear macrumors G3

    Jul 23, 2002
    Sol III - Terra
    I'm not sure the Fusion technology can handle 3 drives properly. And using 2 SSDs for a fusion drive doesn't make sense as it would be extra overhead for no real gain.

    My suggestion is to leave your iMac intact so you don't invalidate the warranty. I would suggest you put the SSD in an external enclosure - a USB 3.0 one will be fast enough for an SSD. And make the external SSD a separate volume.

    Also remember if one drive in a multidrive volume fails, you will lose your data and need to restore everything from a backup.

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