Cannot read nor write to internal and external HD

Hello. I bought a slightly-used Mac Pro M1. Recently, I attempted to drag and drop a file from an external HD to the internal HD, but I get the gray circle with the slash through it. I do a File/Get Info, but I am not allowed to change Sharing & Permissions for anything ("The operation can't be completed because you don't have the necessary permission"). I've tried a number of different things, including copying a backup of the internal HD to the external HD, but now it's "afflicted" with the same thing: one cannot copy to it nor from it, and it can no longer be reformatted to anything except APFS (it was Mac Journaled, but I thought that changing it to the same format as the internal HD would be helpful, not thinking that the Sharing & Permissions would be copied over as well).

From one of the posts from last year about this, it sounds like what's under "Sharing & Permissions" for these drives is correct (system/R&W, wheel/RO, everyone/RO). Another post said that changes should be made under Sharing. I changed everything I was allowed to there, but the problem still exists.

How can this be fixed???