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Can I use multiple external drives to increase TM capacity?

990 Views 5 Replies Latest reply: May 31, 2008 1:46 PM by Tom Lebsack RSS
musiklov3r Calculating status...
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May 26, 2008 6:30 PM
I have four internal drives with 1.2tB of data in my mac pro. I need to use two external drives of 750 gigs and 500 gigs to hold all of the information. Is there a way to tell TM to use both external drives together to allow this much data storage?

Mac Pro octo 2.8 and Macbook Pro, Mac OS X (10.5.2)
  • deh2k Level 4 Level 4 (1,765 points)
    I think you can concatenate multiple disks into a single volume with Disk Utility. This type of RAID set is called a "Concatenated Disk Set".

    I have never tried it myself.

    Be forewarned that if any of the disks in the set fails, the whole volume will be toast. This is the opposite of many other RAID configurations. But for a TM backup, that may be OK.
    MacBook Pro Core 2 Duo, Mac OS X (10.5.2), My other computer is a Prius
  • Camelot Level 8 Level 8 (45,670 points)
    While you can use RAID 0 volumes as Time Machine destinations, I believe this configuration won't work. IIRC, to use RAID 0, each of the disks needs to be the same size. Therefore your 750GB drive will be treated as a 500GB drive to match the other, smaller drive, resulting in a 1TB volume which is not large enough to store your current data set.

    Even if it did work, you'd end up, at most, with a 1.25TB volume (750 + 500 = 1250). Given that you have 1.2TB of data, that only leaves 50GB of space for incremental changes, which isn't likely to be a lot.

    Given that volume of data I would recommend either getting larger drives for your backup destination or don't use Time Machine - use some other application that can create incremental backups while compressing the backed up data.
    Mac OS X (10.5.2)
  • tele_player Level 5 Level 5 (5,380 points)
    a 'concatenated disk set' is not RAID 0, and though I haven't tried it, shouldn't require use of drives with identical capacity.

    Update: I just tried it, using two drives of different sizes. As I expected, a conactenated disk set produced a volume whose size was the sum of the two dissimilar drives.

    I don't know if TM would use it, somebody else can test that. It wouldn't be my choice for a TM drive.

    Message was edited by: tele_player
    MacBook 2.0GhzCore2Duo, 2.Gig, 160G HD, PowerBook G4 1.5Ghz 15", Mac OS X (10.5.1), Logic Studio 8, Focusrite Saffire
  • Tom Lebsack Calculating status...
    I decided to try the concatenated RAID set before going out and buying a terabyte drive. I used three 250 GB LaCie external FW drives, and ended up with 697 GB. It works...TM thinks its dealing with one drive. I agree its not an optimal solution, but it'll do until I get a big drive. Too bad TM doesn't have a way to support multiple drives independently, though.
    Thanks all for the advice.


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