You should be able to copy the folder where the game files reside to another drive; my folder is here (though your drive letter is probably different): E:\Program Files\Epic Games\Borderlands3
SUPER IMPORTANT! Don’t forget to back up your save-game files too - mine are here: C:\Users\Adabiviak\Documents\My Games\Borderlands 3.
When your new drive is installed and working, you’ll need to re-download the Epic launcher. Run it, and you should see Borderlands 3 in your library but not downloaded. I think you want to start a download first (so the launcher sets the directory and starts grabbing files off the Internet). Then stop the download, remove whatever had started downloading, and dump your backed up game files in there. Then when you ask it to continue downloading, it should recognize that the files are already there, and instead of downloading, it should just start discovering/verifying them.
For the save game files, I think you can just dump them back into the Documents folder (same path as before), and your characters will show up… not sure if you need to start a game to ‘define’ the save game file folders first.