Time Machine won't backup new external drive
-External drive on which all photos stored crashed.
Had been using Time Machine to back up both the Mac and External Drive to a large second external drive
-Learned how to use Time Machine Restore properly, and got files back from backup onto new external drive, PHOTO STORAGE.
-Bought new 4TB drive, properly formatted for MAC (rechecked and reformatted drive after it didn't work with Time Machine), to use as new backup drive since old backup drive as old as the one that crashed.
-Selected new backup drive in Time Machine, under TM preferences. Options, under TM, shows the PHOTO STORAGE drive in light gray (therefore not excluded: LaCie2Jul19). As shown below, it cannot be highlighted or turned dark.
Ran Time Machine backup, and it will only backup the MAC: file it has backed up is 1/3 size of PHOTOSTORAGE and the necessary files are not on Time Machine as they were before.
Any sugguestions? Thanks.