A given image set, say 5.3.4 needs to all be contained on one disk. You can have multiple releases such as 5.1.3 and 5.3.4 on any one disk or on multiple disks but all SMUs, SPs, etc for a given release must be on the same disk as the base release.
Do you have any files on disk0:? Disk0: should only be used for OS files, no tar files, no SMUs, no SPs, etc. Those files should be kept on the harddisk to conserve space.