drbd won't create device if previous partition is on it Command 'drbdmeta 0 v08 /dev/md160 internal create-md' terminated with exit code 40

This is what fixed it:

[root@box13 ~]# dd if=/dev/zero of=/dev/md160 bs=512 count=500

Basically you need to wipe out more than just the 512 byte partition table so 512 bytes * 500 is more than enough to make DRBD happy and think the partition is now empty.

The reason this happens is because it gets confused when there is a previous partition with data on the device you are using.

root@box13 ~]# dd if=/dev/zero of=/dev/md160 bs=512 count=1
1+0 records in
1+0 records out
512 bytes (512 B) copied, 0.241883 s, 2.1 kB/s
[root@box13 ~]# drbdadm create-md r0
DRBD module version: 8.3.13
   userland version: 8.3.16
preferably kernel and userland versions should match.
md_offset 1395733229568
al_offset 1395733196800
bm_offset 1395690598400

Found ext3 filesystem
1363020736 kB data area apparently used
1362979100 kB left usable by current configuration

Device size would be truncated, which
would corrupt data and result in
'access beyond end of device' errors.
You need to either
   * use external meta data (recommended)
   * shrink that filesystem first
   * zero out the device (destroy the filesystem)
Operation refused.

Command 'drbdmeta 0 v08 /dev/md160 internal create-md' terminated with exit code 40
drbdadm create-md r0: exited with code 40
[root@box13 ~]# mount^C
[root@box13 ~]# ^C
[root@box13 ~]# mount
/dev/md0 on / type ext4 (rw)
proc on /proc type proc (rw)
sysfs on /sys type sysfs (rw)
devpts on /dev/pts type devpts (rw,gid=5,mode=620)
tmpfs on /dev/shm type tmpfs (rw)
none on /proc/sys/fs/binfmt_misc type binfmt_misc (rw)
sunrpc on /var/lib/nfs/rpc_pipefs type rpc_pipefs (rw)
[root@box13 ~]# dd if=/dev/zero of=/dev/md160 bs=512 count=500