if your xbox is error 21 ... just use XBoxHDM with a softmod installer for XBoxHDM like kingroach's or krayzie etc thats works with XBoxHDM ... this will require hotswaping so theres a chance you could damage pc or xbox or both although i did it many times and never had a issue myself.
if you do this u got to do stuff in a certain order or you WONT be able to communicate with the xbox's hard drive.
also it's a good idea to have dashboard 5960 onto the XBoxHDM cd ... this way once u get access to the xbox's hdd u can delete ALL files on C and restore the dashboard from the cd then run the softmod installer from the xboxhdm bootable cd which runs from your pc.
the basic idea to use that is to.... have the xbox turned off then remove ONLY the IDE cable from the back of the xbox's dvd-rom drive... then power it on it will then error 12 i believe... once it gets to this screen boot up your pc with the xboxhdm cd and once it gets through to a prompt and just sits there u then need to remove the IDE cable from the back of the xbox's hard drive and plug in the pc's IDE cable (the one thats running xboxhdm) into it then hit enter it should then start booting up the xboxhdm stuff to detect your hdd etc etc... theres a little more to it than this but this should atleast get you so u can communicate with the xbox's hard drive... once u do this the rest aint that hard.
IMPORTANT!!! ... also make SURE that you DONT unlock your xbox's hard drive with the XBoxHDM program... if you do your pretty much screwed since you said u currently dont have your EEPROM file.
p.s. all of the above is 100percent free!
p.s. this is referring to your stock hard drive! , the one that error 21's on you.... once you do what i said above this should get your xbox up and running perfectly normall again.... once it's running i would strongly recommend backing up the eeprom file (u can run evox to do this) so then in the future if u want u can then use another hard drive of your choice but u will need to make sure the hard drive your getting supports locking.