A EROFS will be raised when trying to write a dir onto a
read-only filesystem. However, if the dir already is there,
then it should be treated like an EEXIST (since EROFS can
be raised by trying to create a dir over a mount point,
where the mount point is not read-only, but you still can't
just clobber over it).
If the dir doesn't already exist, then the EROFS will be
accurately reported as the reason why it could not be