I had this problem some time ago, because they’re set up for windows. If you have access to windows it is a simple repair and you will be able to view everything again in Linux systems as well. I did it this way. 1. Plug in your external hard disk .
2. Open File Explorer by pressing the Windows keys button + E.
3. Go to This PC.
4. Right-click your external hard disk under Devices and drives .
5. Select Properties.
6. Go to Tools tab.
7. Under Error checking, click Check.
Let it do it’s job and then remove it and everything will be okay.
You totally resolve this from happening again, but only do this if you really have your files safe. (I have 4 back up drives) by formatting the drive so that will only accept linux files, think that EXT4 but check and then just use it for that. I have one that I just use for Mint. I did the above first then transferred the files I wanted to my laptop which had Mint on them and then transferred them back to external drive. It is a bit of a pain to do it, but I’ve not had problems with either since.