We've had good experiences with the 2.5" SAS drives from Seagate. I think they're around 10K RPM. I believe they are the ones Dell uses in their "Green" servers.
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The 2.5" HDDs makes me wonder about IO speeds. Surely these are not server grade drives?
2.5" is a common size for SAS drives. They're even faster than 3.5" drives due to there being less distance that the drive heads have to move, and they use less power due to having smaller platters to spin. The ability to fit 24 drive slots on the front of a 2U server (or 8-10 on a 1U) is also very nice.