Welcome to the community, @andiwerewolf!
I'd recommend you check how the HDD appears in Disk Management. Posting a screenshot from there might be pretty helpful as well. If you haven't tried this already, try using a different USB cable and another USB port on your computer when connecting the external hard drive.
If you are able to see the external HDD in Disk Management, you might also want to
try changing the drive letter . It could help you get the HDD properly recognized and accessible through the file explorer (My Computer/This PC).
Keep me posted & Good luck!
SuperSoph_WD