Comment 26 for bug 1374759

Hm, so while in Xenial we did move to os-prober 1.70 (the Debian bug was closed by 1.68) it seems the check for it might be too late (50mounted-tests sound like a place that will never be reached as any attempt to mount the extended partition will be unsuccessful, of course *after* all the scary messages emitted during the mount attempts).

I believe the following addition directly in os-prober will do the trick. At least it seems to be good in local testing.