If I mount it, will I essentially format it? I'm asking because I'm running a node and I don't want to lose my syncing so far or I need to know if I should back up the blocks in case something happens.
I would personally recommend to backup the blocks directory to another drive, wipe the drive, format it as ext4 and put back the blocks. Then do a reindex.
Also, I wouldn't recommend to
(allow all kinds of access to all users), instead simply change the ownership of the folder to only the user that runs bitcoin core:
sudo chown -R username /media/external/bitcoin