I've been through a few iterations to troubleshoot this. It only happens periodically, like once a day. I've tried the seagate 160 that shipped and also tried a WD250GB drive and it happens on both. I did have the macpro loaded with 160 + 250 + 250 + 250 drives when it started happening, but am since running just 1 drive at a time til i get to the bottom of it. Basically i'll get the beach ball and i'll hear the drive head click and then the drive spin up again. It'll take about 15 20 seconds and then things will come back to life. I've tried the power management with HD parking on and off, doesn't seem to affect it. It's very irritating, and i'm stuck with the next step. If it were a hw issue i'd expect it to happen quite regularly. Any advice is gratefully accepted. M.