07 is hdd timeout. Open it up and check none of the cables have come loose. If they are all fine, then the hdd is probably dead.
If you have an eeprom backup (I'm guessing you don't as you say you were going to mod it), then just get a lockable hdd and build a new hdd with xboxhdm and lock it once you are done. No eeprom backup, then you're going to have to hard mod it (chip or tsop flash), so you can use an unlocked hdd in it.