You didn't mention which version of NetBackup you are using.  At least 
if you are using v5.1 or older, catalog tapes have a different format 
than NetBackup data tapes.  If you try to write data to a catalog tape, 
you receive the infamous "it contains VERITAS NetBackup (tm) database 
backup data and cannot be used for backups."  

For this reason, I have a pool just for catalog tapes.  My catalog tapes 
do not go back to Scratch, as this will just gum-up the works.  When one 
of my catalog tapes expires, it just sits in its pool waiting to be reused.

As you have seen, "frozen" tapes stay frozen forever.  Justin showed you 
the way to "unfreeze" one of these frozen tapes.  This makes the tape 
available for use and discards any catalog entries for data that might 
be on the tape and not yet expired.  Because a frozen tape has been a 
party to some sort of "bad event", I try to understand the problem and 
address it.  For example, I allow the data on the tape to expire, 
because it might be useful one day even though the tape has participated 
in a bad event.  Once expired, I run bpexpdate to remove the frozen 
status causing the tape to appear unused in my catalog pool (or data 
tape to revert to my Scratch pool).  I immediately do a "quick erase" or 
"long erase", else if I don't have time, I change the tape to a separate 
pool used just to hold the tape until I can erase it.  If the erase goes 
well, the tape I give the tape a second chance and change it back to the 
pool from whence it came.

You didn't mention your NetBackup server operating system, but on 
Unix-like systems, there is a file important to the process of finding 
and handling bad tapes and tape drives:

    /usr/openv/netbackup/db/media/errors

There's one line in that file for each tape event (not all events cause 
a frozen tape).  I err on the side of caution and if a tape is involved 
in an event, I freeze it (see the manual for syntax) and handle as 
above. Note that a tape can't be frozen if (1) it is currently mounted 
or (2) has no unexpired data.   If a drive seems to be involved in 
several events, I cause it to be cleaned (More than once if I'm 
considering service to the drive).  Warning: don't use your cleaning 
tapes for more than your drive manufacturer recommends.

I'll end by noting that the verbose version of the "Media Summary" 
report shows "frozen and expired" tapes ... that is, tapes that you or 
NetBackup have frozen and all of their image data have expired.

Hope this helps!   cheers, wayne