There seems to be a problem creating shadows of the drive objects using the setting SHADOWID=x:. Therefore I strongly recommend to attach an object ID to a drive object before creating a shadow of it. (see also Creating drive objects)
In OS/2 WARP 4 all drive objects have an object ID by default. The format for the ID is <WP_DRIVE_x> where x is the drive letter (A, B, ...).