Hey there!
Can you say what the drive is used for? E.g. Time Machine, or just as a manual backup drive, and/or if it’s also used with other OS’, E.g Windows, etc.?
Likewise, can you say the name of the brand of external drive you’re using, and list the available storage and partitions, if any?
A picture of Disk Utility, while the main external Disk and external Volume, (Make sure to select View > Show All Devices, while in Disk Utility, to see all), are selected will help!
Likewise, if you highlight the external drive in Finder, press Command i on the keyboard, and expand all the sections, especially the Sharing and Permissions section. That may help.
Other good tests:
Does the same happen on another Mac? (If able to test).
Does the same happen on another port/cable?.
Was it working fine one day, then just stopped working suddenly? Or were there any changes made from when it was working to not working?
From what you describe, it’s unpredictable behavior which makes it tough to diagnose, but also wondering if the same happens to files transferred from / to different locations on the drive.
Likewise, perhaps, it’s your admin user profile that’s causing the problem, or even third party software you have installed.
Fir each scenario, testing from another user account, and/or safe boot is a good test/
https://support.apple.com/guide/mac-help/start-up-your-mac-in-safe-mode-mh21245/mac
Sorry, just spitballing ideas, I’ve seen a lot, let me know, happy to help if I can!
