Is there steps that are specific in ugrading a HP-UX media server to 5.1 from 4.5? I know in Solaris you have to remove the package etc before the upgade and specify that it's a part of an upgrade.
Nothing special, I did that on a couple of media servers, it worked fine.
Just to be sure, your master must be at least 5.1 mp5 before, and when you stop netbackup on the media server before the upgrade, check that nothing is there with bpps -a.