time machine backup failure

Don315 wrote:

'Time machine couldn't backup to 'G-drive mobile' because it is 'read only'. I don't know how that can be as backup has been working till a week ago, then this message.

How is that Backup drive formatted...?

Command i opens the information pane on a selected drive:

NTFS requires software to make it r/w on the Mac, ex Tuxera, Paragon—there are others

If you can't back up or restore your Mac using Time Machine - Apple Support

https://support.apple.com/en-us/HT204157

*Trouble Shooting—

https://support.apple.com/guide/mac-help/time-machine-troubleshooting-mh15653/mac

Don315 wrote:

'Time machine couldn't backup to 'G-drive mobile' because it is 'read only'. I don't know how that can be as backup has been working till a week ago, then this message.

If you have been successfully backing up and now it won't and suddenly the drive is "read only," that is sometimes a symptom of a failed or failing external drive. Check the external drive with Disk Utility, try to repair it with First Aid. It that does not work or it persists in being "read only," the disk is likely failing. When the MacOS detects a failing drive it makes it read only and instructs you to copy what you can from the drive. I recommend obtaining another backup drive, formatting it APFS GUID, and trying a new backup with it, as soon as possible. Right now you might not have a reliable backup.

This happened to me a while ago with a backup drive. Since then, I use two Time Machine backups, Time Machine alternates between them, each drive has its own self contained complete Time Machine backup of the Mac. As further insurance you can make a "clone" backup which you can update periodically. Yet another layer of protection is to store another b backup (any type) in a separate location or in the cloud.