Many people have went to the live section in games with their modchip on and didn't realize that MS sent some info to your xbox that now gives you a Error 21 when you try to go to your msdash (but evox still works fine). In most cases you just send your backed up C: from your computer back to your xbox. But i have read a few threads where people didn't and they think they are screwed. Slayer's new autoloader 2.5 has a option to replace/fix your msdash. This will indeed fix this error 21 but after it fixes it you need to rename the new xboxdash.xbe to whatever matches up in your evox.ini file.
-Silicone

I had the same problem and not knowing how to resolve it at the time renamed my msxboxdash.xbe to xboxdash.xbe to try and cold boot straight to MSdash thinking that it may get the dash working properly.
I was sure that I could boot the xbox with Slayer's boot disk to return it to modded condition. Unfortunately my xbox can't boot from CD and is stuck on error 21. I also have lost the precious C & E: backups with no unlock code of the standard RETAIL HD. Apart from being screwed and stuffed can this conditon be recovered from without the need of a new drive.