Time Machine + 2 external HDs

[waiting_for_OSX]

I have Time Machine running on an external USB HD. If I buy another external HD and plug that into my firewire port will Time Machine do incremental backups of the firewire HD in addition to my primary system disk?

[JeffCGD]

So it won't back up the Firewire hard drive. An alternative solution would be to use a product like SuperDuper or CarbonCopyCloner which can be set up to do incremental backups of any specified volumes (for two volumes – your Boot volume and your Firewire volume – you would set up two scripts). Note that SuperDuper can clone your Boot Volume to the same volume as your Time Machine backup – and will give you a BOOTABLE backup, which time Machine does not provide. I would recommend partitioning the USB drive as your backup (as it's the slower of the two drives) – one for Time Machine/SuperDuper backups, and one to backup the Firewire drive.