One thing I found out with the ProLiant ML350 G6 that I own is if you use the built in SAS/SATA backplane it can be very fussy with consumer level drives. The "HP" branded drives have a tweaked firmware with regards to temperature monitoring and some models of consumer drives report back as being overheated when attached to the backplane. It causes the fan to spin up to "jet engine" levels almost constantly.
Don't know if the G9 line has that same issue or not.
I found a bunch of good info on this issue here http://dascomputerconsultants.com/HPCom ... Drives.htm
In the end with my machine I just removed the backplane, bought a SAS expander that hooks up to the onboard P400i SAS controller and attached multiple SFF-8087 to sata cables to it which have worked perfectly. I don't hot swap drives with my server but I do have 14 drives in it right now.