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 ~]# d........
I've only used it on Centos, soI thought I'd make a quick Debian guide:
Install the DRBD Package
apt-get install drbd8-utils
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
libswfdec-0.8-0
Use 'apt-get autoremove' to remove them.
The following........