Question: Does anyone know if I can install normal 2.5" SATA drives inside an HP DL360 G5 server? Right now it has 6 72GB SAS drives. I am looking for raw storage for my files SAS performance in not required.

Answer: 
SATA2 disks should be compatible with the Smart Array P400i that comes with your server (or any other SAS controller, for the matter), although probably not with stellar performance as others have noted. But keep in mind that not all SATA disks are suitable for use with RAID because of the infamous Time-Limited Error Recovery parameter (TLER for WD drivers, also called ERC by Seagate and CCTL by others). Disks not prepared for RAID configurations will frequently drop from the array.
Few times I had a problem with SATA drives inside Proliant servers where under Linux drives were detected as PATA.

The solution was to turn off Auto mode for hdd in BIOS settings...