What generation is the ML370? Up to G4, they took what Compaq/HP called 'Universal' drives, which were Ultra2, Ultra3, Ultra160 or Ultra320, and certainly went to 300g, but they cost a friggin fortune (like around £800 a piece!).
Later ML370 (and other HP hotplug servers) use what they class as SFF drives (basically 2.5" similar to laptop), which come in SATA (entry level) or SAS (enterprise level), and are more energy efficient. They just released 500G SAS varients.
All ML370s were hot plug, so won't go into other options.
Be aware some SMART controllers cannot create logical drives bigger than certain sizes - if yours is a ML370 G1 (ie, beige in colour), you've probably got a SMART 3200, which I seem to recall did have limits.
If you don't have a SMART controller, and are running off the onboard SCSI, get a SMART from egay (dirt cheap now due to age) - SMART 6400, SMART 5300, and SMART 3200 will all work well (performance in that order).
I have a few 18G universal drives knocking around which you are welcome to.
You ideally need the SMARTSTART CD for your server, if its a G1 as I suspect, from memory the V5 ones are they ones that are compatible, pretty certain the V6 ones don't support 1st generation ML370s. I have a decent selection of SMARTSTARTs at work, as quite a big download from HP, and not all the older ones are available.