I have been trying the MechAssault game save hack on a virgin 1.0 XBox without success. I was using a "Nuby" brand memory card. Once "Run Linus" is selected, the service screen appears, with flashing red/green LED.
So, I experimented with a chipped XBox (X2.2Pro), and to my surprise, the hack did not work either when the modchip is disabled. But with the modchip enabled, the hack works. I FTP'ed the game save files from E:UDATA... to the PC. File comparison with winhex reveals that all files are identical to the original zipped files, except for default.xbe, which is different at offset 0x100 to 0x1FFF. Suspecting that default.xbe was somehow "unsigned", I directly FTP'ed the original (from zipped file) default.xbe to E:UDATA... and the hack works.
Without going into too much details, I discovered that once default.xbe is copied into the memory card, it became unsigned. This happens whether I FTP'ed the file directly to the memory card, or copied from the hard drive using M$ dash. Hypothesizing that maybe the game controller was responsible for this, I hooked up the memory card to a different controller. But I have the same result.
In conclusion, the "Nuby" memory card unsigned the default.xbe file and circumvented the game save hack. Now, if I can get a hold of an original M$ memory card, I can test to see if the hack will work as the above posts indicated.