Backup drive formatting changed?

Recently Time Machine has stopped backing up to my Seagate external backup drive, although it still shows up on the desktop and is mounted. Error message says it can’t write to the drive but that the backup data is accessible and I should save to another drive. I see that the Seagate is formatted in HFS+ and not writable. I believe a backup drive must be formatted as MacOS Extended (Journaled) to work. The drive has worked with TM for years — why would it suddenly not be formatted correctly? Would a recent High Sierra update have done that? Thanks for any help!