Since it wasn't backed up, how important is your data? You can run recovery tools on one hard disk, but don't do it on the other one if losing the data isn't an option.
You can try http://www.partition-recovery.com/products.htm#file_recovery, http://www.easeus.com/datarecoverywizard/recover-lost-partition.htm
and other suggested tools. I'll admit that I never had much success with free recovery tools; that's why I perform backups. However other mambers may be able to suggest a free utility that will undo the damage done by Clear MBR. All data should still be there; only the partition should be gone; so fixing the MBR may work.