Most files are invisible on my WD My Passport Ultra

For several months I have been ripping my cd’s, putting them on two my Passport Ultra HD’s. This morning, on one of those HD’s only the most recent files were visible. The other files must still be there, since the occupied volume is still 1.1 TB, as is the case on the other HD.
How can I make those files visible again? And what can be the cause?
The HD has not been dropped or exposed to heat, water or anything;
Thank you!

Hello Clemrobyns,

It might be possible the files are hidden and you should try searching these hidden files using Windows hidden files search option, if using Windows.

OK, I’ll try that. Thank you.