diff --git a/lib/geom/label/glabel.8 b/lib/geom/label/glabel.8
--- a/lib/geom/label/glabel.8
+++ b/lib/geom/label/glabel.8
@@ -172,6 +172,15 @@
 and
 .Pa /dev/label/bigdiskp1a .
 .Pp
+.Nm
+requires write access to a device to create, modify, and destroy
+"automatic" labels. It will fail to persist changes if they are
+applied while file system on the device is currently mounted. To ensure
+that modifications are stored across a reboot, the file system 
+needs to be unmounted first. If the device hols the system's root
+file system, entering into single user mode and mounting in read-only
+mode is required to persist the change.
+.Pp
 The first argument to
 .Nm
 indicates an action to be performed: