The time machine is not backing up a external hard drive connected to my imac - how do I fix this?

I have 3 Sandisk 1TB external Hard Drives. One is showing up in the Exclude These Disks window and is greyed out so I can't select it to delete it from the list and have it backed up. The other two aren't on the list so I assume they're being backed up but I don't see anything in the TM backup disk that suggests they're in fact being backed up.

How do I:

1. Get the disk that's greyed out in the exclude list to be included in my backup - again I can't select it so I can hit the - key to remove it from the list
2. How do I confirm the disks I "think" are being backed up are in fact being backed up?

SOLUTION:

I copied the contents of the external SSD drive that wouldn't let me delete it from the Time Machine "Options/exclude from backup" dialog" to a new location, re-initialized the external SSD disk in a different format (Mac OS Extended (Journaled)) copied the original contents back to that external SSD drive and was then able to select/delete from the time machine/options dialog which included it in the backup.

Open time machine preferences

Select options

Ensure the disk is not excluded

The external disk may not be correctly formatted.

Welcome to the Apple Support Communities.

Was Time Machine ever backing up to an external disk? Do you have an external disk connected to your Mac?

Jack

Yes I mistakenly excluded the disc - how do I in-exclude it. When I try to back up now it asks me to erase & I can't see the previous back ups on there

yes