Creating multiple Time Machine backups on a single disk attached to AirPort Extreme

... so that I can create multiple Time Machine backups for different machines on the same disk, ...

You can certainly do that but I recommend dedicating at least one hard disk drive for each Mac that is going to be backed up. You will most likely need a powered USB hub due to the power limitations of the AEBS's USB port: Uses for the USB port of AirPort Time Capsule, AirPort Extreme, AirPort Express - Apple Support

Speed is not important with Time Machine. Redundancy is.

If you do this then make sure that it's not your only backup for each machine. One dead disc would mean losing backups for more than one Mac. Each Mac should have its own backup at least. Minimum recommendation is 3,2,1 - three backups, 2 different methods (e.g. TM, plain copy) and 1 kept offsite.

Each Mac that backs up to the drive connected to the AirPort Extreme will automatically create its own Time Machine sparsebundle file for the Mac, with the name of the Mac assigned to the sparsebundle, so you can identify which file is which.

So, you can have multiple Macs backing up to the hard drive at the AirPort Extreme.

Note that the hard drive connected to the USB port on the AirPort Extreme must be formatted as Mac OS Extended (Journaled), also known as HFS+.

All that you have to do is set up each Mac to back up to the hard drive connected to the AirPort Extreme and the rest will take care of itself. But, don't try to set up all of the Macs to back up at the same time. Set up one Mac to back up and let the first backup complete before you set up and run the backup for the next Mac, etc.

The only real downside to this is that each sparsebundle will automatically expand as more files are added, so you cannot place a given limit on the sparsebundle size. Basically, it's first come first served as fare as space will be concerned on the hard drive at the AirPort Extreme.

Another possible downside is that the USB port on the AirPort Extreme is an old...and slow....USB 2 design, so backups are not going to run all that quickly.

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