Unable to access External drive for Time Machine backup on Macbook

I connected a seagate external drive to backup my Macbook.

It backed up a few times.

Now it has started showing me a TimeMachine error saying that TimeMachine does not have access to the drive. I have tried going to the Drive Info using Cmd-I, unlocking the padlock on bottom right. But the gear icon is greyed out and I am unable to change any permissions on this drive! See screenshot below.

How do I fix this issue and start backing up my files?