I had softmodded a friend's Xbox months ago. I was going to re mod it using the ndure exploit, but before I had a chance to call him, he called me telling me he was getting an error 21 message. Well the machine won't cold-boot Mechassault at all. I had the eeprom backed up, so I took out the hard drive, hooked it up to my PC and ran hddmaker. I put the C drive back into a virgin MS state. This method has always worked for me before, so I wasn't worried about the situation. I plugged it back into the Xbox and I got a 21 message again. I formatted the drive again and hooked it back up to the Xbox and I still got a 21 error. This is ticking me off.
This is the disgusting part, when I opened the Xbox I discovered that a roach had laid eggs in the machine at some time, and there were about 20 dead baby roaches in the machine. I figured the heat killed them. Some of them were stuck to parts. Could these bugs, somehow have damaged the motherboard??? He told me he bought the system used, so beware of this potential problem in the future.
He said he was playing Ultimate Spider-man one day, then he turned the machine off. The next day he got an error 21 when he booted up. I've played Ultimate Spider-man on my machine using the same mod he did and I didn't get this problem.