hi rob.
ideally, your sample drive should be separate from your system drive and your audio drive. if i had a choice, i'd separate ivory as well, coz ivory itself is already taxing on your hard drive, assuming you're sequencing music that is piano-heavy.
well, either that or increase the buffers on ivory to relieve your hard drive a bit. but then you'll want to upgrade your memory if you decide to go this route.
kitc is right (he always is... haha). you should put your samples in an internal drive, but separate it from the system drive. i have samples on my system drive, but they're the ones that i use least often, and those that load to memory (as opposed to streaming directly from the drive), like atmosphere libraries, trilogy, stylus, sampletank libraries, etc.
i'd use an external drive for audio, coz it's easy enough to change hard drives when i run out of space for other projects. that way your sample drive is left alone.