Long file name problem: copied to external drive but can't copy back

Welcome to the forum :)

File name too long usually results from the file being buried too deep, that is, folders within folders. If this is the case I suggest you try using Everything Search by VoidTools which is totally free (donations accepted) Fast and efficient, I have been using it for years and have never had a problem
Everything Search has a portable version so you can test it without installing it. I always use the portable version

• Using Everything search for a folder about half way down the tree of folders within folders
• Right click on the folder and select Copy
• Quickly open Windows File Explorer and paste the folder anywhere you like for the moment.
• Open the folder and make sure the copy was a success
• Use Everything again, right click and Delete the original folder

Hopefully you will now be able to copy the other folders to the C drive, then organise copying the other folder across but not to the same position or you will get the same problem again
If the copy does not complete then you need to go deeper down the tree so you are copying less files and remember to copy then paste quickly

Note: Never Move the files, if something goes wrong they can be lost, always Copy then Delete

By the way, I did have the path too long problem and this did work for me, took a few attempts but I gradually chipped away at it until the path was finally under the limit

Good luck :)