If you mount the LVM device under a DRBD volume instead of the DRBD one itself, then DRBD does not track any changes you make to the disk.  As a result, it will not synchronize those blocks to the secondary if you later switch back to using the drbd resources.

To force a full verification and resync, add a line like this to the resource config:

 syncer {
        verify-alg      md5;

Then run:

drbd adjust r0
drbdadm verify r0
## Watch the verify process in /proc/drbd
## if the oos:XXX value is greater than zero, blocks are out of sync
drbdadm disconnect r0
drbdadm connect r0

it will only force the resyncing of blocks if you disconnect/reconnect.