howto/ganeti: how to detach a disk

howto/ganeti.md

@@ -702,6 +702,14 @@ never do that.
 
 See also the [upstream procedure](https://docs.ganeti.org/docs/ganeti/3.0/html/admin.html#conversion-of-an-instance-s-disk-type) and [design document](https://docs.ganeti.org/docs/ganeti/3.0/html/design-disk-conversion.html).
 
+### Detaching a disk
+
+If you need to remove a volume from an instance without destroying data, it's
+possible to detach it. First, you must identify the disk's uuid using
+`gnt-instance info`, then:
+
+    gnt-instance modify --disk <uuid>:detach test-01
+
 ### Adding a network interface on the rfc1918 vlan
 
 We have a vlan that some VMs that do not have public addresses sit on.