The missing partition issue is similar to what happened to me.  When I tried to mount it on a linux system one of the 3 partitions that I had on my 750GB hard drive wouldn't mount.  I ran the linux file system repair utility against that partition (e2fsck) and it said that the partition had a missing or corrupt superblock.  I let the repair process run on the drive and it was able to fix the superblock issue and also fixed a lot of other problems it identified with the partition structure at the same time - it ran pretty much all night long but when it finished I was able to reconnect the drive to the dvr and access my media.