👍🏽 It's always difficult to know where to start to help since we aren't looking over your shoulder.
Since you've already done all that, the best I can suggest is to copy everything else off the drive to another, and then erase and reformat this problem child.
You said you deleted "an old backup... from an older Mac... that stopped working" so I wonder: just how old is this mechanical drive?
If it's more than five or six years it may be showing it age and just needs to be retired.
Consider running Drive DX from Binary Fruit. It's a pretty thorough disk diagnostic utility. I believe there's a free trial version that can answer the majority of questions about the drive and point out any deficiencies.