Hi,
Frozen media is not used by Netbackup for backups (you can restore though) as it has either been frozen for a reason by the system, or it has been manually frozen (I am guessing you have done this based on how you think it behaves).
You can manually freeze media as a way for Netbackup not to use tapes, but lots of other ways exist. In your post you are correct on how frozen media will behave (apart from the images portion) but your confusion might be why you have a frozen tape.
Media can be frozen for a number of reasons normally "Media" Issues.
These are normally write errors, media positioning errors or allocation errors. In some cases it can be media mis-match errors (i.e. The barcode does not match the electronic version on the tape, the type of tape does not match how you have it defined etc). It has Netbackup Catalog data on it, is write protected etc). The list is very big I am afraid.
I think Technotes exist for the reasons and they are pretty extensive. The above are from personal experiance.
The bottom line is:
If you have frozen media they will not be used for backups (Netbackup marked them in the Media Database for Saftey reasons as it thinks a problem exists)
They may have an allocated time and date (as that's when Netbackup went to use it) but NO images as it did not get as far as putting any data on the tape.
If you have images on the tape, it is more than likely that you experianced some media write/positioning errors. but remember
just the MEDIA has been Frozen, not the images on the media so they will expire as they would on a "good" tape.
If you want to keep the images safe as they were written with the wrong retension or you have other reasons you can use bpexpdate to change the retensions in the database for the images.
If you only have just one frozen tape, it is quite likely it is just a "dodgy" tape or perhaps its as simple as the write protect tab is on.
Hope it helps.