Third Bootcamp back failed now target drive Read only and no name

My 3rd backup failed writing to its usual external drive (1TB USB). All the contents are there but no name, just an icon on the desktop. It cannot be renamed as it’s read only now for some reason. I’m happy to Erase and try again but can’t remember which format to choose in Disk Utility, is it DOS?
I have had 2 previous successful backup attempts.

EDIT: OS 10.11.6

When you say “backup”, do you mean that you did a disk to disk clone or that you saved the image to the external drive?


Thanks Tim
I did a disk to disk clone, which worked fine the first 2 times.

Thanks. You can also format at HFS+ and save an image, rather than disk to disk clone.


Yes indeed, thanks Tim. The external drive I was using is fine as it’s now storing the disk image. So I still don’t know why it failed using disk to disk clone. However thanks to all who responded.