Drive with Final Cut Pro video files on it is dying, need to point FCP to the backup drive
Reconnecting the original and even proxy media is very easy in Final Cut Pro.

Disconnect your oldDrive
Connect your newDrive
Open the Final Cut Pro X library and open the media tab. All media which can't be found will look something like this:
In your top menu bar go to file, next reconnect media and then select original media.
Search for the files on the newDrive.

This Apple support article covers this also very well.