Moving Steam to an external hard drive. Unplugged during transfer
Solution 1:
I'm assuming Windows 10. Your files only are deleted from the source when the move procedure is finished. So, wipe external HDD, and restart.
If you don't wipe the external HDD, you'll be prompted to overwrite. Chose yes to everything, and it should work as well. Just make sure you chose yes when overwriting, since one of those files is definitely corrupted since the transfer was interrupted.