How cold boot different a warm boot?

A cold boot always begins with the power off whereas a warm boot does not interrupt the power supply. Although there may not appear to be any difference on the surface (a POST is performed regardless), a cold boot is more reliable as it ensures all hardware is properly reset.

When diagnosing hardware issues, it's always best to cold boot the system to ensure consistency. It's also best to let the system rest for at least 30 seconds to allow any residual current to drain before powering up again. Ideally, external peripherals (USB drives, printers, domestic routers, etc) should be turned off as well. Hardware issues are problematic enough without increasing the number of variables you have to contend with.