What should be done first if a hard disk does not work after upgrading the power supply?

If a hard disk does not work after upgrading the power supply, the first step should be to ensure that the power connectors are properly plugged in. This is crucial because during the power supply upgrade, it is possible that some cables were not securely connected or were accidentally disconnected. Checking the connections will confirm that the hard disk is receiving the necessary power to operate.

Compatibility of the hard disk is important, but if it was previously functioning, it is unlikely to be the immediate issue. Replacing the power supply again is a more drastic measure and would not be logical if the problem could potentially be resolved with a simple inspection of the connections. Formatting the hard disk is not relevant in this scenario; formatting is only necessary if the disk is recognized but has data issues, which is unrelated to the power supply upgrade situation. Thus, starting by ensuring that all power connections are secure is the most logical and practical first step.