After you've attached a new CD-ROM dirve, go into the Prefs drawer, run CDDevice and find both the device and the unit number.
Make sure your CD0 driver in DEVS: is set to this unit number and that the DEVICE entry is pointing to the correct device
e.g.
/***************************************************************/
/* CacheCDFS mountlist entry © 1993 Elaborate Bytes, O. Kastl */
/***************************************************************/
FileSystem = L:CacheCDFS /* The name of the game */
Device = "atapi.device" /* Name of exec device driver */
Unit = 2 /* exec device unit */[/size]
Flags = 0 /* OpenDevice flags */
BlocksPerTrack = 351000 /* Unused */
BlockSize = 2048 /* True, but unused */
Mask = 0x7ffffffe /* Memory mask for direct read */
MaxTransfer = 0x1000000 /* Maximum amount of bytes for direct read */
... etc
