Scott,
A 3rd Harddisk only needs an IDE-port OR a free SCSI-connection..... IRQ's are not needed.

Please realize an operational HDD can be destroyed by system-failures or virusses....just like all other data-storage devices connected to your computer. An infected 'backup' is NO backup !!

I still prefer backups on a (closed session) DVD, because virusses cannot affect them. If your pc gets on fire ...a 3rd drive does not help much

Roel