Final answer:
The best initial troubleshooting step after BIOS does not recognize a hard disk following a power supply upgrade is to check the power connections to the drive. If that's not the issue, the next step would be to check the SATA cable connection, before considering updating the BIOS or hard drive replacement.
Step-by-step explanation:
If the BIOS doesn't recognize one of the hard disk drives after upgrading the power supply, the BEST troubleshooting step to try first would be to check the power connections to the hard disk drive. This is because if the power supply has just been replaced, it's possible that some cables may not have been reconnected properly or may have come loose during the process. Moreover, checking power connections is a simpler and less invasive step compared to updating the BIOS or replacing hardware. If the power connections are secure and the issue persists, the next step would be to check the SATA cable connection. It's uncommon to need a BIOS update for a power supply change or to have a hard drive fail at the same time as a power supply upgrade, making these options less likely as the first troubleshooting steps.