Help please with xbox

hi everyone, could someone please help me with my xbox which is softmodded. on switch on x box screen comes on then changes to service required and error code 21 in top left hand side, i can put an original game in tray power off and power on and original game will play, but no backup will play, i therefore need it to read my slayers disc to try and sort out my problem and i am therefore asking for help for my x box to read my slayers disc.thank you for taking time out to read this.
cheers,
grimmers

Can you FTP into it??

never tryed ftp have no idea how to , all i know is to connect x box to pc with crossover cable.
cheers,
grimmers

Taken from Xbox-HQ

Service Error 21
Info: This error says that the machine was booted to display a error, basically someone told the machine to reboot (or launch a xbe) with this flag, and the error code just means its been rebooted by the flag. This error seems to show up more often when hacking/patching/customizing the original dashboard. E21 (Error 21) is a dashboard related error.
Normally it means you are missing vital files from your xbox c:\ parition or you are running the wrong M$ Dashboard for your xbox version.
E20 is directly related to the Dashboard files but i have found from previous experience E21 is very simlar. Even though you get E21 you should still be able to BOOT GAMES from the DVDROM drive… The bios boots the DVDROM first and if no dvd found in the drive it will boot the xbox c:\ parition next.
Solution: You probably are running the wrong dashboard for your xbox version. Use the ‘HQ Auto Installer Deluxe’ to reinstall your Retail M$ Dashboard… It should fix this error no matter which xbox version you have… If not try downloading another Retail M$ Dashboard.
Related topic(s) on Xbox-Hq.Com:

Basically you need to return the system back to retail state and get the softmod re-applied, or try to gain access via FTP and sort the problem that way. Other than that your pretty much stuffed unless you get a modchip fitted or a TSOP flash, which can be removed afterwards if you wish to keep it softmodded. I don’t think gaining FTP is an option due to no dashboard present and backup media not booting.

Guys i also have a problem, i have the error message 16 when i turn my xbox on. I unlocked my hdd but i dont know how to put anything on it. My xbox is not modded at all and i cant play games on it or anything. Can somebody please help me?

Error 16 is the clock problem. You deleted the dash and now the clock cannot be set.

regards, Stephen