The 50 pin older SCSI drives are the cheapest although they are not easy to find in larger sizes. Segate Cheetah (10K rpm)or Baracudah 7200rpm) drives are the best overall but price can be a bit steep, used maybe around $60 for a 10 gig. They use either the 68 pin or 80 pin cables so make sure you have the right adapter. SCSI is definatly the way to go for the Amiga. And gives you up to 3 times the speed over IDE.
