- The manpage for umount says that -R can only be used with
a mount point (e.g. /usr/local) and not a device name;
this makes sense because a device might be mounted in multiple
locations, but the mountpoint (and things mounted under it) lives
in the filesystem tree.
- Existing code tried to unmount -R the device, not the mount point,
and so always failed; leaving things mounted that shouldn't.
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