I came across this problem on another forum (advised the poster to come here for help) but it's been niggling me since I read his description of the problem.
1). Softmodded Xbox (SID/AID) 200GB HDD everything working, not sure about the dash being used. No Shadow C I'd guess but could be wrong.
2). The guy lent the Xbox to a 'friend ' just to play some DVDs (allegedly) but after received back found it was only booting to the MS dash.
3). He tried using his previously working AID disc and received a disc read error message.
4). The original Mechassault exploit/installer game saves are still on the HDD so next he runs the game. It loads OK so disc drive problems, my first thought, seem to be ruled out and with (3) this suggests the softmod is gone.
5). However the installer dash refuses to load and after a few seconds returns him to the Mechassault game menu.
6). Solution: he deletes the existing Mechassault game saves and, presumably from a working USB stick/memory card, copies in "new" game saves.
7). Tries Mechassault again - same negative result.
That's where he is now.
It sounds as if he knows what he's doing - he has EEPROM backup and the original retail softmodded HDD available. I think he probably also knows how to sort this out himself too but doesn't want to go through rebuilding the HDD again.
The question he's asking and I'm interested to know too is what could be the cause of the installer dash refusing to boot?