pull out the drive, stick it in a pc and run some diagnostics on it, if the S.M.A.R.T system is tripping up on it, something may have happend.
When your kids opened the system to clean it out, did they wear anti-static protection? Like a grounded wrist strap?
Did they pull on any of those small ribbon cables inside? I would take it back apart and ensure all the connections and wires are properly seated, none of the wires of the modchip has come loose, the network/HDD controller is properly connecting to both the motherboard connector and the hard drive, and there is no loose screws rolling around in there.
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