I have 2 x 500 GB external Harddrives and 500 GB internal harddrive . I use time machine to backup from the internal 500GB and one external 500GB HD to the second 500 GB HD. This is good till date since I only had to back up less than 250 Gigs in total.

What would happen if and when I exceed the disk space of 500 Gigs?

Can I use another 500 GB harddisk (in order to horizontally scale) on Time Machine . I need incremental backups like TimeMachine does and I like the interface too. I picked out 500 Gigs as an example.

  • Can I mount 2 external HDs as a single one so that the TimeMachine does not need to know about it ?
  • Does TimeMachine support backing up to use multiple harddisks depending on space constraint ?
  • How is this problem usually addressed for a non-business requirement ?


3 Answers 3


Yes - there are two ways to do this.

Apple intends and designed Time Machine to work with disks in a sequential manner. You are expected to fill up the first disk and put it on the shelf. Then attach the second and fill it up. You can then go back to the first and this lets you have more history before the oldest backup is reclaimed due to space running out.

An alternative is to use Mac OS X built in RAID functionality. You will have to erase all the disks when you put them into a RAID the first time - but you can have two disks with half the data written to each every time a file gets written. This is striping or RAID 0.

Using a RAID would mean both drives need to be connected all the time. You would then have one large backup and since Time Machine won't be saving two copies of many files - you will get much longer time for storage using a RAID than the first method of using the drives sequentially. If your boot drive is almost full - I've seen times where the first method could store a month or two of history, but years would fit on the second method. It really depends what files you have and how often they change.


When you fill up a Time Machine disk, the Time Machine software starts deleting the oldest backups to make room for the new. I haven't heard it suggested that Apple expects you to put the old drive on a shelf when it gets full. At any rate, I have about 350 GB on my main hard drive and 500 GB stored on my Time Machine backup--and it goes back almost exactly two years. I can live with two-year-old backups getting deleted.

Another strategy that I have heard is to have two backup target disks and switch them every week (for example), so that if either one is lost or fails, you've lost relatively little. Time Machine will work with this strategy.

A RAID 0 would create a bigger virtual disk, but the problem with RAID 0 is that if either disk fails (and with two disks, your odds of failure are doubled), all the files are lost, so this is not a great backup strategy. RAID 0 is mostly used to increase disk speed, not to increase capacity. A similar but slightly less risky approach is to concatenate or span the drives into one virtual drive. In this scenario, if one disk fails, you lose the files that were on that disk, so it's still not a great backup strategy, but you can set up a spanning disk volume in Apple's Disk Utility.


I've used Apple Software RAID-0 with four drives for Time Machine since 2007. It's true that with two drives you have roughly twice the probability of failure. If you take a very very small number and double it (or in my case quadruple it), you still have a very small number. I haven't had any failures.

In addition, we're talking about backup, not primary storage. If the array fails, you fix it and start over. There's a tendency for people who haven't had a backup for ten years to treat their new backup as if it were live data – it's not, it's just a backup.

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