Corrupted GUID partition table on secondary hard drive
Boot to your OS X system
Open Terminal and enter diskutil list
Enter sudo gpt -r show diskX with X the device number of your external disk found in the previous output (probably 1 - below I assume it's 1 but use the disk number found in your environment in the commands below)
Enter diskutil umountDisk disk1
Remove the one and only partition:

sudo gpt remove -i 1 disk1

Add the EFI parititon:

sudo gpt add -i 1 -b 40 -s 409600 -t C12A7328-F81F-11D2-BA4B-00A0C93EC93B disk1

Add the main partition:

sudo gpt add -i 2 -b 409640 -s 3798935504 -t 48465300-0000-11AA-AA11-00306543ECAC disk1

Verify disk and volume:

diskutil verifyDisk disk1
diskutil verifyVolume disk1s2

If you get an error in the last two steps add a comment.