BTW, you can build standalone backup on another disk, or another root on your system disk. It is much faster than booting from tape.
When it boots standalone backup, even from your system disk, it copies it into memory,
(a RAM disk) which can then backup itself.
You can permanently change the sysgen parameters for that root.
You should know that for later versions of VMS, stand alone backup is no longer supported, and the use of a CD is recommended.
CDs are cheap. And, they are much safer to work with, since you get a minimal amount of VMS, so you can check the backup, mount a new disk and perform various operations, such as anal/disk/repair on the disk.