500 old media, or 50 old media?
Yes, I'd create dummy media entries on the new master - and place the dummy media entries in to a 'do_NOT_use' volume pool. You could also write protect all of the old media. Do this whilst you consider your options for the long term...
If you ever want to restore from them in the new environment - then clearly you'll need to know the contents. So, if you want to decommission the old env - then it's usually best to import the old media - or to have previously engaged consulting services to migrate the NetBackup Catalog. I dare say they could still do it. But you cannot do this.
So, you only options are:
1) Leave the old environment up. And place dummy media entries in the new env so that old media do not accidentally get overwritten.
2) Create and save and copy a media and catalog listing of old env. Shutdown old env. And hope that no-one asks for a restore. And if they do, then import as quick as you can the tapes that you think you need (from what you can discern/decypher/gather from the old listsing).
3) Import all the old media in to new env. Then shutdown old env.
4) Engage consulting to migrate/merge the remnants of the old catalog in to the new catalog, then shutdown old env.
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Questions:
1) Were the old media encrypted?
1a) If so - can you re-create the keys, or export them?
2) What type were the old media ? (LTO1/2/3/4/5/6? or something else)
3) Do you have a compatible tape drive on the new master server environment (either on master, or on a media server)? (i.e. what are the drive types in teh new environment).
3a) If not - and assuming the old library is FC/SAN - you could re-zone the old tape library to the new master or a media, and then import using the old tape drives?
3b) If not, and you do NOT have a compatibile tape drive - then you'll never be able to read the old tapes until you acquire a compatible drive type - in which case you're probably stuck with the old env until all old images have expired naturally.