Friday Q&A: Restricting Time Machine’s Size

Q.  I’m a big film fan and I spend a lot of my time porting over films to my Mac and its external hard drive. It’s great because I can watch movies on the go without having to lug around an ugly black CD wallet. However, I’m also running Time Machine into this external hard drive, and if I’m not careful I’m going to run out of space. Is it possible to control how many backups Time Machine makes onto the external hard drive? I don’t know if I need the data from way back in October.
Daniel Holt

A. Time Machine stores backups differently according to whether the backup disk is connected via your local network or directly attached using USB or FireWire. Networked Time Machine volumes are created as sparse image bundles, which have their maximum size set when the bundle is first created. This lets you fool Time Machine into using less than the whole disk by putting large files (such as non-sparse disk images) on the disk before you turn on Time Machine for the first time. Once the sparse image bundle has been created, you can remove the padding files and the Time Machine backup won’t grow beyond this original limit. On locally attached disks, though, Time Machine backups use ordinary files and folders, and the backups will keep growing until they fill the entire volume. The solution is to make sure that the volume is smaller than the disk, which you do by partitioning that disk.

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This question originally appeared in issue 224 of MacFormat, the UK’s best selling Apple magazine.

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