It's always recommended to use a 7200RPM drive for the samples in the following order:
1) Non-system non-audio SATA internal
2) Non-system non-audio SATA external
3) Non-system non-audio FW800 external
4) Non-system non-audio FW400 external
5) Non-system non-audio USB external
6) System drive
Start at the top and go down the list and select the option closest to the top which works for you.
Cheers,
Rail