It is frustrating when an external hard drive becomes read-only on Mac. If you are using the drive formatted with macOS-supported file format when it changes to read-only, you can try to repair it with Disk Utility or reformat it to make the drive writable again.
Incompatible file format is the most common reason when an external hard drive is read-only on Mac such as NTFS on Mac. Given that you have "Used for ages only with my Mac with no problem at all," the culprit shouldn't be the file system.
Other causes for Toshiba read-only on Mac could be the drive permission and drive errors. To solve external hard drive read-only on Mac, you can change permissions on Mac in the first step. Right-click the drive and choose Get Info, under Share & Permissions, change your account's privilege to Read & Write, if you can see the Ignore ownership on this volume, check the box.
If the Toshiba drive is still read-only, you can try to repair drive errors with Disk Utility's First Aid feature. It can check and repair logical errors on the drive. Just head to Finder > Applications > Utilities and open Disk Utility, select the external hard drive, and click First Aid to start the process.
Unluckily, the hard drive remains read-only, you may need to format the external hard drive on Mac to gain full read-write access. However, it will erase all the contents, you can restore it from a backup if you have created one, if not, try to recover deleted/lost data from the drive with iBoysoft Data Recovery for Mac.
