Dying HDD or Catalina?

Hi All

I've got an external 3TB G-Drive which I use for time machine and I've recently updated to Catalina so I'm not sure if the OS is the problem as I've read quite a few articles about TM issues with OS 10.15.7.

Anyway I always shut my Mac down at night, but lately when I turn it on in the morning my backup drive goes into this 2-3 hour task of doing something, but I've not idea what. I thought it was preparing the TM back up, but even if I cancel the backup it still carries on with its task. And when the task has finished and I turn TM back on everything seems to backup fine.

I've checked activity monitor but can't see much running, apart from a kernel task which is reads about 10GB an hour at a ridiculous slow speed of between 4mb and 1.68mb/s. It seems like spotlight is trying to index it or something but apparently spotlight doesn't index TM drives in Catalina.

Is there away to see what process is actually using the external HDD or does it sound like a failing drive?

I've also had a look in console but to be honest I've no idea what I'm looking at, there's just a million requests for diskarbitrationd going off.

Any ideas or things to try would be greatly appreciated before I shell out on a new drive.

thanks