Seagate External Harddrive is now read only since Ventura 13.0 update!!

What is the drive used for? If it's for Time Machine it is a read only drive for the user.

Have you tried to bring up the Info pane for the drive and change your permissions to it to Read & Write?

How is the drive formatted?

If in doubt: select the drive in the Finder, press Command-I

and post a screenshot.

Time Machine does create a Read Only drive when it first uses it.

Try holding down the Option keys when accessing the Time Machine menu and select "Browse Other Backup Discs". Then select the same disk and see if it will use it.

Thanks all - erase and reformatting did the trick.

I'm surprised you're getting that warning because Time Machine uses Read Only with APFS format.

Do you have any files in the TM drive backups that you don't have on your Mac? If not then I'd erase the drive, format it as APFS and set it to be the TM drive again. The first backup will take longer.

I have the same issue as the OP. The external drive was used for Time Machine and as soon as I upgraded to Ventura, the Time Machine backup fails. Error says: "Files can’t be copied onto the backup disk because it is read-only."

I think this was a bug that come in with the Ventura update as it clearly happened as soon as I upgraded. In the end, I was only using for time machine backups so I could afford to reformat the disk and recreate the backup from scratch, which I've now done

I have the same issue. And the last time machine back up has not occurred since Nov 4.

Do you get any error notifications?

Yes - says TimeMachine couldn't complete backup to "Seagate Backup Plus Drive". Files can't be copied onto the backup disk because its read-only

Thanks - I'll give that a try later tonight!