BIOS
The Basic Input Output System, or BIOS, on the system is used to load the operating system on the console. The BIOS.bin file is housed in the "E" folder and is used to boot the BIOS on startup.
Disk.bin
The Disk.bin file is used to load the operating system off of the hard disk on your console. Many people FTP into the console and backup the Disk.bin file to replace the original hard drive with a new one. The Xbox comes with an 8GB hard drive. The capacity is too low for housing emulators and music.
EEPROM.bin
The Electrically Erasable Programmable Read-Only Memory, also known as "E2PROM," is used to erase the memory on the drive's cache. This file is housed in the "E:" folder on your console.
HDDinfo.txt
The HDDinfo.txt file on your E: folder is used to log hard drive errors or failures on the system. When you FTP into the console, you will be able to load the text file on your computer and see if there are errors on your drive.