rsync, USB Drive, Error 30: "Read-Only File System"

I was regularly copying a folder to a USB drive in Ubuntu using rsync.  Suddenly, for no apparent reason, the rsync process started producing these error messages:

rsync:  failed to set permissions on [USB drive/folder/filename]:  Read-only file system (30)

rsync:  mkstemp [USB drive/folder/filename] failed: Read-only file system (30)

rsync:  failed to set times on [USB drive/folder/filename]: Read-only file system (30)

rsync:  delete_file:  unlink [USB drive/folder/filename] failed: Read-only file system (30)

rsync error: some files/attrs were not transferred (see previous errors) (code 23) at main.c(1058) [sender=3.0.5]

The fix, for me, was to reformat the USB drive in Windows.  It would probably work even if Windows was not the cause of the problem.