File comments in Get Info are missing. How do I recover?

Notes that I added to file comments under Get Info have gone missing.

There are hundred of files and they contain valuable copyright information.

A Spotlight search using a couple of people's names I do recall bring up several results, but still leaves 99% of files with empty comments fields.

And after typing-in new info into the Comments field and ejecting the external drive in which they are saved, when opening that file again, comments are gone.

How do I recover comments that clearly are still in the file but invisible.

And more importantly, how do I stop this from happening?