How to know what is on my external drives, when they are not connected?

You may take a look at the application called NeoFinder. It was designed for exactly this purpose. It has existed for a very long time, and is available for mac on Apple Silicon or Intel, and is also available on iPad and iPhone.

I have not used it because I don’t have that many drives, by I have known about it since it used to be called CDFinder.

Here are a few apps that you may find useful for cataloging your drives and other media:

• NeoFinder
• Nifty File Lists
• Print Window

As Luis said. I have used CDFinder/NeoFinder and BBEdit > Edit > Insert > Folder Listing...

Giving each drive a unique name (with logic that identifies it) has always been my solution for multiple drive identification.

For example: "Bailey's SD! Bootable Clone". A "Bailey " is a variety of apple fruit and my 2015 21.5" iMac Named Bailey has a SuperDuper! clone attached. The drive for non essential archival data is "Bailey's Data Drive". There are others similarly named.

I can look at an any drive and know instantly exactly what the drive is for, what type files to expect and they're always delineated by descriptive logical-use names.

Each of our five Mac's has multiple external drives, there are at least three each, and all can be immediately identified.

Not possible at all! Your CAChe would have to be huge to reach old hard drives through the Years!