Recently I had to upgrade to OS X Lion; given that I didn’t want to lose all the backups performed since my first days in OS X’s world, back in April 2008, I studied for a while a plan that allowed me to move a Time Machine disk across.

I know, I know: had I used Migration Assistant, there probably would have been no issues at all, but I moved things manually and took the time to clean up a bit some things. Anyway, moving such a disk is not difficult, really, it just takes a lot of time.

What?!?!?

Before going on, however, maybe it’s better to clarify what I mean with “move”: I don’t refer to moving the machine directory from a smaller disk to a larger one (another easy operation: fire up Disk Utility and “restore” the smaller disk onto the larger one; you’ll end up with a “clone”, just bigger), I mean using the same machine directory with a differt machine, keeping the history and list of every backup already performed.

With Lion, there’s no need to fiddle with ACLs, extended attributes, MAC addresses and so on, thanks to the addition of tmutil; nevertheless, there will certainly be some troubles due to “insufficient” Spotlight’s indexes that will slow down backups if we don’t improve the situation (would you like if your backups lasted hours and you logs filled up with tons of Waiting for index to be ready (100)? No, I guess the answer is no).

Please, note that the following operations are potentially destructive, will prevent you from using again the backup disk with the old Mac, should not be performed if you’re not comfortable with the terminal and you don’t know what you’re doing. Moreover, in no event I shall be liable for any claim, damages or other liability. If you accept that risks are all yours, go on.

How?!?!?

Let’s assume the following:

• backup disk is called MyBackupDisk;
• the Mac is called MyMac;
• old Mac’s disk is called MyOldDisk.

So, without further ado:

• turn off automatic backups (sudo tmutil disable);
• connect the backup disk;
• associate that disk with the Mac:
• “reset” Spotlight’s indexes:
• reindexing everything will take a lot of time, so you’ll have to wait patiently;
• turn on automatic backups (sudo tmutil enable).

With brand new Spotlight’s indexes, Time Machine won’t have any problem to quickly backup your machine and all your old backups will be at your disposal.

Before ending, let me repeat: I have personally executed this procedure successfully, but it involves direct manipulation of your backups, so take your time to think about it before doing anything and be ready to take the consequences.