Yes, here is more specific information:
The Computer: 2017 iMac / MacOS Catalina 10.15.7 / 40GB DDR4 /
I have been using time machine to back up my current iMac since purchased in 2017. I am using two naked 8TB Western Digital 3.5" Drives with an OWC Dual Drive Dock to have a redundant backup of my iMac. One of the Drives (The "A Drive" back-up drive has 401.11 GB of 8TB remaining.
This "A Drive" back-up Drive is failing to back up and displays the following notification message:
"Time Machine couldn’t complete the backup to iMac Back-up Drive"
"The backup disk needs 1.05 TB for the backup but only 401.54 GB are available. Select a larger backup disk or make the backup smaller by excluding files. Last successful backup: Februrary 25, 2023"
The 2nd "B Drive" back-up drive has 519 GB of 8 TB remaining. It is backing up as normal. My iMac has internal 1 TB SSD storage.
Time Machine has been backing up to both back-up drives as normal with this set up since I set it up 3 years ago.
I performed Disk Utility's First Aid on th "A-Drive" back-up drive to see if there is a problem with the back-up disk or catalog. These are the results:
Running First Aid on “B-1 HD Time Machine iMac Back-up Drive, ©2020 Daniel O'Connor” (disk6s2)
DISK UTILITY RESULTS:
"Repairing file system.
Volume was successfully unmounted.
Performing fsck_hfs -fy -x /dev/rdisk6s2
Checking Journaled HFS Plus volume.
Checking extents overflow file.
Checking catalog file.
Checking multi-linked files.
Checking catalog hierarchy.
Checking extended attributes file.
Checking multi-linked directories.
Checking volume bitmap.
Checking volume information.
The volume B-1 HD Time Machine iMac Back-up Drive, ©2020 Daniel O'Connor appears to be OK.
File system check exit code is 0.
Restoring the original state found as mounted.
Operation successful."
In System Preferences, Time Machine states the following:
Time Machine keeps:
Local snapshops as space permits
Hourly backups for the past 24 hours
Daily backups for the past month
Weekly backups for all the previous months
The oldest backups are deleted when your disk becomes full.