If you're worried about maxing out a firewire connection, a USB HDD won't be able to deal with that deluge of data. Besides, six 96kHz channels with 24-bit data is only ~1.7MBps, which isn't much.

Take with a grain of salt, and try somewhere like http://hydrogenaudio.org/

BTW, could you elaborate on the "1-2 channels being recorded and about 10-15 being played back" part?