Some people have noticed an issue in Snow Leopard where iTunes will query for Audio CD track information and then promptly unmount the CD from the system.
Apple: Improper unmounting can cause disappearance of files in Mac OS X 10.4.x
Mac OS X 10.4.4 Special Report: Acard-connected drives unmountable, other issues
Mac OS X 10.4.4 (#11): Acard-connected drives unmountable, other issues; Fujitsu MO drives -- attaching to a hub; more
FireWire-based RAID arrays spontaneously unmounting
Mac OS X 10.4.2 Special Report: FireWire Drives spontaneously unmounting
Mac OS X 10.4.2 Special Report: FireWire/USB volumes refusing to unmount
Devices refusing to unmount: Force quitting the Finder may resolve
More on drives refusing to unmount; solution from Symantec
Devices refusing to unmount: USB 2.0 also affected by Norton AntiVirus?