This is a little How-To from oops-ups I have experienced.
Linux
gfs2 and drbd
Recovering files on gfs2 on a broken drbd
First: Bring up the cluster
drbdadm up <cluster name>
If the local node is secondary make it primary
drbdadm primary <cluster name>
Then disconnect it from its peer
drbdadm disconnect <cluster name>
Finally mount the file system. Note that mounting read-write did not work for me
mount -o localcaching,localflocks,lockproto=lock_nolock,ro <device> <mount point>