Using Time Machine after a major OS upgrade

Question

One of my colleagues had issues on her MBP recently, and I had to take a look at it. It turned out that the machine was still running Leopard and had a bunch of junk installed on it. For the sake of security I upgraded the system to Lion and placed her documents to where they were, using her Time Machine backups.

Everything seems to work fine, tho I couple of questions arose:

• Will Time Machine work as it is? (it's on an external disk with only one partition)
• Will she be able to browse through her old backed up files while running the new Lion system?

My guess is that the Lion backups will not be compatible with the old backups, as in she will not be able to "continue" on the old backup. However I don't see why it would be a problem to have two different sparsebundles in the same disk. As for "entering Time Machine" from Lion, I'm not sure if that will work as Time Machine seems to use one sparsebundle for the backups and I haven't seen any ways of selecting a sparsebundle.

Can anyone confirm or deny these assumptions?

Solution

If the Mac still has the same name, the same hard drive and the same logic board, then Time Machine will not have any problems identifying it as the same machine again.

This means that your Time Machine backup will just continue to write to the already existing one. Of course, this results in a much larger initial backup since many system files need to be changed. The backup may take a very long time. But apart from that, the previous backups should be preserved.

If that's not the case, Time Machine will ask you for which drive to back up to and ultimately create a completely new backup. The old one will be preserved and accessible though – it won't be touched again.

It really depends on what you want to do. I personally would just keep the old backup archived somewhere and start a new one.

---

If your Mac continues to write to the old backup, note that old and unused files can be deleted from the volume. For example, at some point, Leopard's system files won't be used anymore and therefore removed from the backup. Your private documents though won't, if you're using them.

Nevertheless, Time Machine should work fine with older backups. In fact, there haven't been any (major) changes to Time Machine's file layout ever since it was introduced. However, it's not backwards compatible, therefore you can't use the backup on a machine with Snow Leopard or Leopard.