Q&A: Time Machine and NAS

Q How well would Time Machine back up to NAS?

A When Time Machine first appeared in OS X, it was very picky about the types of drive which it would back up to, and most NAS drives did not work at all, apart of course from Apple’s own Time Capsules. Since then Time Machine has become less picky, and NAS vendors smarter about making their systems work for backups.

To work with Time Machine, a NAS now has to convince OS X that it is running the Mac Extended (HFS+) file system, and of course has to be mounted at startup. Several NAS manufacturers claim that their products are compatible with Time Machine, so if you have one of their NAS devices you should follow their instructions.

There is a generic method which should work with any NAS, detailed here. The plan is simple:

  1. create a sparse disk image using Disk Utility, in HFS+ format;
  2. copy that disk image to your NAS and mount it;
  3. set Time Machine’s pane to make its backups to that mounted disk image;
  4. set the disk image to be mounted during startup.

However some users report persistent glitches and issues when trying to use some NAS which claim to be compatible with Time Machine.

Using a NAS successfully may not always be a good idea either. During the initial complete backup, and any subsequent large increments, backing up over a network is likely to slow other network activity, hog the NAS itself, and take a long time. If an error does occur, the masquerade may fall to pieces and you may find it impossible to recover your backup from the NAS. As backups need to be as reliable as possible, you may feel that you are better off using an external drive array instead.

Updated from the original, which was first published in MacUser volume 26 issue 4, 2009.