Could be ur drives are on 2 sata ports that are a different color to the others as they're shared with esata ports with a separate controller. (these are removable by default)
..... the drives are setup as dynamic volumes (or whatever the selection is for hot-swapping drives). Computer->Manage->Drives can show u this and have options for changing.
..... ur sata controller of choice as used by the mobo people sets all drives up as hot-swappable. In conjunction with a raid setup maybe.
Maybe a few other reasons too. Laptops have a habit of doing this with optical drives, often as they`re designed to be unattached whilst Vista is still running to change for harddrives and such.
Key thing is being able to unplug them whilst the pc is running.
You definitely can't eject/disconnect the drive that is running Windows. Thankfully it's intelligent enough to throw an error when you attempt that. Not sure about non-OS drives tho.