Anyway you seem to know what you are doing if you do it for customers, you should at least

Guess you will have some spare 360s then, go put the drive in there and check if it works or gives the same symptoms....
If it does the drive is fucked if not it will be the 360 or one of the cables.
Since you have heatgunned it it might be the Southbridge so get a voltage checker and make sure that the power connector is powered.
If it is not it is definitely the Southbridge, then you will have to reflow it with the heatgun as well
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