@nemoide, If I recall correctly it does essentially erase the HDD. You might be able to transfer to USB (via controller port) if it's file size related.
@nightrnr, I seem to remember thinking there's a catch to hot swapping HDDs with nulled keys & it's more intended for easier rebuilding of drives... but now that I'm thinking about it, I can't recall exactly why maybe I'm wrong
Also as a caveat, the crew rebuilding the OG XBLive (project Insignia) has come out & said the HDD/EEPROM key is used as part of XBL auth. There's no way around it. So if you want to hop on Project Insignia later when (if) it's released, need to not have it nulled.