Q Can I install a 2.5” SATA hard disk on one of the sleds in my Mac Pro? The connectors all line up, so it seems physically possible. Could I then use that disk to augment the existing volume used as my Time Machine backup?
A Provided that the SATA connectors fit your drive, it should fit and work in your Mac Pro, although it is unlikely to perform as well as a regular 3.5” SATA drive.
However there is currently no way of using it to expand the area available for Time Machine backups. Those backups must be performed to a single volume, and you cannot spread them across a virtual disk composed of two or more volumes.
Some file systems such as the superb ZFS, which almost came to the Mac, can create virtual drives from multiple volumes, but the closest that Mac Extended (HFS+) comes is with software RAID mirrors. When you designate a new volume as the destination for Time Machine’s backups, the first thing that it does is perform a full backup to that volume, which would consume much of it before any further backups are made.
Comments There is no sign of this inflexibility on the part of OS X being changed, so long as it sticks to the Mac Extended file system, I am afraid.
Updated from the original, which was first published in MacUser volume 28 issue 10, 2012.