This would explain my issue as well.

Main machine has 3 SATA drives, 36gb Raptor (System, Sata port 6), 320gb WD Cav (Dump, Sata port 4), 320gb Seagate (Dump, Sata port 2). Installed Vista x86 onto the Raptor, wont boot unless CD is in.

However, I have a second machine with 2 x IDE (120/200) and a Sata 250. Installed onto Primary IDE Master and it boots fine.

Gonna move my drives around when I get home tonight and test. Hoping this is the fix!

And yeah Scotty, XP doesnt have this issue. Bit of a pain :<