Cartridge cases have about the same physical dimensions, but the physical tapes and the recording technologies differ. In the following cases, the drive is not capable of reading existing data, but it can read an existing volume serial number and the drive allows software to write over the data with a different recording technology, for example:

• 36-track tape mounted on a drive that is only capable of handling 18-track tapes
• 256-track tape mounted on a drive that is only capable of handling 128-track tapes
• EFMT2/EEFMT2-track tape mounted on a drive that is only capable of handling EFMT1-track tapes

In these cases the open or EOV functions can retrieve the volume identifier. When the first block is written on the tape and as an IBM standard or ISO/ANSI standard volume label, the tape subsystem extracts the volume serial number and saves it in an area accessible only to the drive. When Open or EOV functions later issue a read for the volume serial number on specific drives that cannot read at the recording technology, the I/O is unit-checked. If the device supports the 'vol in sense' capability, the VOLID is retrieved and inserted into the sense that is returned on the unit-checked I/O. If VOLSNS=YES is coded in the active DEVSUPxx member, Open/EOV will pick up the VOLID from the returned sense.

The OPEN or EOV functions rewrite the tape with the drive's recording technology, possibly without standard labels, if all of the following are true:

• The application program is opening to write to the first data set
• The volume serial number is available from the vol ID mark or from the external label from the vision system
• DFSMSrmm, the label anomaly installation exit routine, or the volume label editor installation exit routine allows the rewrite
• The user has RACF UPDATE authority to the volume or RACF is not controlling tape security
• The DEVSUPxx member of SYS1.PARMLIB specifies VOLSNS=YES