If you’ve closed the file, and just want to save your original edits, you could move the file back to your desktop and work on it from there.
To fix the external drive problem:
Could be the path is too long, or has invalid characters. You could try renaming the folder(s) on the external drive where the file is, shortening the folder names in the path, or the filename, or the drive name, look for odd characters in the path, leading or trailing spaces in the folder names in the path to the file, etc.
If you’re on Mac, maybe an ampersand in the drive name? Or leading or trailing space in a folder name?: