Time Machine say it couldn’t complete back up because disk is nearly full, but it isn’t

Time Machine says it cannot complete the backup because the disk is nearly full, but it is not. In fact, the Time Machine tab under System Preferences says “2.68 TB available”.

Time Machine was working fine for a couple of years, then suddenly started getting this error. I have not made any major file or fold moves recently.

The external backup drive is 4TB G-DRIVE which has always been used exclusively for Time Machine backups. I noticed from the beginning that this single drive shows as two different mount points. Under “About This Mac / Storage”, it shows as:

External 4 TB USB Disk

G-DRIVE mobile USB-C
2.86 TB available of 4TB

G-UTILITIES
4.04 GB available of 4.08 GB

I assumed this was normal but perhaps it is not. If it’s not correct, I don’t know how to correct it.

G-UTILITIES just contains few instructional documents that came with the drive. They are about 30 MB total in size.

Under “About This Mac / Storage”, my hard drive that I back up (Macintosh HD), shows “1.52 TB available of 2.12 TB”. That is, it is about 71% full.

What do I need to do get the backups running again?

(I see other related posts on similar issues, but they are not quite the same. Most of them refer to Time Machine becoming confused by large file or folder moves which is not relevant in this case. And none of them address the 2 mount points for the single backup drive.)

Please see 4 screen shots below.

Thanks!

Time Machine error message

System Preferences / Time Machine tab show plenty of space

About This Mac show main hard drive and single backup drive with 2 mounts

Both drives as viewed from "finder" app.