Move Time Machine backup from local disk to Airport Extreme disk and share with others

Solution 1:

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

Solution 2:

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.