Unmount stale NFS mounts

If you have a stale NFS mount hanging on your system it can cause various programs and utilities to fail. A typical symptom is a hang when using the 'df' command.

In such cases you cant do umount /path/to/stale/nfs because it will say "the device is busy" or words to that effect

To fix this you can unmount it with the 'lazy' option;

umount -l /path/to/stale/nfs

If you don't expect that mount point to ever be available again (for example the nfs server was decommissioned) then make sure you adjust /etc/fstab accordingly.