Your eprom is the locking number that is on the motherboard and on the harddrive. Each eprom is unique with its xbox.(however there are way's to flash your eprom to use a different harddrive). You could not lets say take your friends unmodded xbox harddrive that is locked with his eprom and stick it in your unmodded xbox and expect it to work. It would either give your error 5 or 6(cant remember exactly).
Now that you know what an eprom is, you should start to understand why its so important(without it, if your xbox harddrive gets messed up and you cant unlock it(everytime time you shut down your xbox it relocks the harddrive unless you have used a tool or dashboard loaded onto the xbox to basically tell your xbox harddrive not to lock when it looses power) it is a very complicated proccess to gain your eprom back(however with a couple days of work you can get it back).Now the catch is that an umodded xbox will not run an unlocked harddrive.
Thats where the modchip comes in(a softmod will not run with an unlocked harddrive becuase you are still loading a bios of of the original xbox... so if you have a softmoded xbox then DO NOT UNLOCK IT UNLESS YOU NO EXACTLY WHAT YOU ARE DOING!!!!! I am telling you this from exprience... Once again it can be fixed but takes alot of unessesary work). A 'hacked bios' is capable of loading a unlocked harddrive, so lets say that you and your friend both have modded xbox's (remember hardmodded) and want to switch harddrive's. All you would need to do is you would both need to unlock your harddrives and then simply switch them(remember:if you are not trading for good, then you 1. cant access the original dash and 2. relock your friends harddrive in your xbox, b/c you would then lock your eprom onto his harddrive(which if you do then it is a simple as unlocking the harddrive in the xbox that you locked it in and then putting it back in his xbox and relocking it).
To find you eprom, you must first have a modded xbox(this can be a softmodded xbox or hardmodded because you are not locking or unlocking the eprom when you do this) and assuming that you are using evox as a dashboard, ftp(if you dont know what ftping is, then search for it) into your xbox and find the file where you specified the 'hacked bios' to find evox. With evox there should be a file named evox.ini On your desktop, create a folder named temp. Then drag the evox.ini into that folder. Now open it with notepad and go almost to the very bottom until you come to something like this
[Menu]
Section "Root"
{
Item "Play Game or Media from DVDROM Drive",ID_Launch_DVD
Section "Play Games loaded on HDD"
{
Line "- Games on the Hard Drive -",2
AutoAddItem "E:\GAMES\"
AutoAddItem "E:\HDDLoader\"
}
Item "DVD MOVIE Player - Region Free","e:\Apps\DVDX\default.xbe"
Item "Xbox Media Center - XBMC","e:\dash\xbmc.xbe"
Item "Avalaunch Dashboard","e:\Apps\Avalaunch\avalaunch.xbe"
Section "System Management"
{
Item "Manage HDD Games with PxHDDLoader","e:\Apps\HDDLoader\default.xbe"
Item "DVD2XBOX Game & Media Backup Tool","e:\Apps\DvD2XboX\default.xbe"
Item "Choose a Different Skin",ID_Skins
Section "Launch Apps from HD"
{
Line "- Media and File Managers -",2
Item "Dvd2Xbox Media Copier","e:\Apps\Dvd2Xbox\default.xbe"
Item "PxHDD Loader","e:\Apps\HDDLoader\default.xbe"
Item "Xbox File Explorer","e:\Apps\boXplorer\default.xbe"
Line "- Auto Added Items -",2
SortStart
AutoAddItem "e:\Apps\"
SortEnd
}
Item "MS Dashboard - Use to change Time/Date and TV Settings",ID_MS_Dash
Item "System Settings",ID_SettingsIt may not look exactly like this but it will look similar. Now edit the last part of the file(the bold part) so that it now looks like this(however you do not need to actually make the text bold)...
}
Item "MS Dashboard - Use to change Time/Date and TV Settings",ID_MS_Dash
Item "System Settings",ID_Settings
Item "Backup",ID_BackupNow go up to save as in notepad and save this file as evox.ini on you desktop(NOT in the temp folder you created). Now after you have edited that file, ftp the file that is on your desktop(the one Not in the temp folder) to the location of where the original one is located on the xbox. It will ask you if you want to overwrite say yes... after it is finished transferring the file, turn your xbox off and turn it back on... and the click on system management and at the very bottom of the list you should see a little icon that says Backup... click this and you have successfully backed up your eprom!!! go back to your computer and ftp back into you xbox, and look on e:/ you should now see a folder that says backup, this is your eprom. Copy the entire folder over to your pc and put it in a safe spot. After you have your eprom onto your pc, go back to where your evox.ini file is located on your xbox and dump the file from the temp folder back over to your xbox. You can then delete the temp folder and the file that you created on your desktop. Congrad's... you now have your eprom backup an should fully uderstand what your eprom is for and why you need it. If you have any questions then pm me and I will answer them the best I can...
I would also recommend you read about xboxhdm
here(its how to use your eprom if you were to mess your xbox up and it also explains how to upgrade your haddrive a little bit better)
p.s. Sry about all the typo's... 3:00 in the morning and no sleep... lol