If the MCPX were faulty I would not expect the console to be capable of booting to an error screen.
1.0 consoles won't throw any error codes unless you modchip or TSOP flash them.
Usually an error screen will point at either the HDD or the files on it. In rare occasions, the HDD will fail to spin up due to a faulty power supply.
To test the PSU, just hook the HDD up to your PC's power supply (leaving the IDE cable connected to the X-Box motherboard), start the PC, then the console. If it works normally like this then the console needs a new supply installed.
There's also a (very slight) chance that the IDE cable is faulty, or even that the socket where the cable connects to the motherboard might be loose. Easy enough to test these things; try a different cable, and reheat the relevant solder joins on the underside of the motherboard.
If none of that stuff fixes it, then it's highly likely the HDD is at fault. It may be that the drive isn't physically broken, but the files on it are just messed up; either way, an EEPROM reader should allow you to sort things out.