I did my tests on the limits and i found 2tb is the end of the line under os9. Even with my fibre connection os9 chrashed when i bumped the workspace up to 3 or just slightly above 2tb.
Besides for efficent recording you should use either a scsi raid or firewire drives which can be newer or even a sata card in the machine with added drives. Protools seems to be a headache when it comes to drives. It has to be internal. There are few versions that let you record external but thats on avid hardware. Protools unity.
I haven't played around with a SATA card, but others have pointed out that depending on one's setup, a SATA card might not be ideal or even a problem, as it can hog the PCI bus.
I haven't done much recording, but I've mixed sessions with 16+ tracks with my setup (new SATA drive, SATA <> IDE adapter), and haven't run into any issues. Other than the volume size limit, I haven't had many issues with ProTools at all, fingers crossed.
