Add command to power off external drive

Guides/mount.md

@@ -64,3 +64,11 @@ sudo umount /media/jovian/iss_share
 sudo umount iss_cyph
 sudo cryptsetup close iss_cyph_bridge
 ```
+
+### Power off external drive
+
+Useful for safe removal.
+
+```sh
+sudo udisksctl power-off -b /dev/sdb
+```