I recently picked up a Drobo for storage and light backup, and want to clear the 1TB Western Digital external drive I have off my desk. I would like to still use Time Machine, but possibly off a share from my Airport Extreme, so my wife can use it on her laptop too.

This disk currently has been running Time Machine locally for my iMac. If I move the disk to the extreme, and tell Time Machine to use it - will it just start off from where its at, or will it have to do an entire new full backup and start over again?

And with that, can you have multiple machines backing up to the same disk shared through an Airport? Or should I partition it for the different machines using it?

2 Answers 2


I have run TM backups both locally and across the network.

Moving backups:

Just moving the disk won't work. If you don't need to use the old backups, it's probably worth just biting the bullet and waiting for it to complete a new backup (if you connect an ethernet cable on a gigabit network it's faster (~90MB/s) than connecting the drive via USB (~30MB/s)).

If you need to keep the old backups, you could try creating a sparsebundle with disk utility, mounting it, and then copying the backup files directly onto it. This method would be slower than creating a new backup from scratch over gigabit though, so if you don't need the old backups, it doesn't buy you anything.

Multiple machines:

Yes, you can backup multiple machines to the same Time Machine disk

  • Went ahead and answered last night with what I saw, but this is even better and actually answers the rest of my question. I just fired my iMac backup off an hour ago, and its going through wire (though, not gige b/c my Verizon router is in the way). It worked just like this. Didn't even try to mess with the sparesebundles and save the old backups. Reformatted old drive, added it to Airport Extreme, and just worked.
    – jmlumpkin
    Apr 14, 2011 at 12:06

According to various sources, this is not possible since how Time Machine works differently between local drives and network shared ones (one reference on Ars Technica).

When you backup to a local hard drive, Time Machine just makes a bunch of folders for the backups. When you backup over a network, it creates a sparsebundle (for reasons that are extremely critical to allowing Time Machine to work reliably over a network), and stores those folders in that sparsebundle.

Since I knew I didn't have much of anything to loose, I just reformatted the drive and started it again.

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