Just because you put +5V, +12V, and ground on the Molex plug doesn’t mean the drive is going to be powered up and spinning. The controller on the drive controls the motors (duh) and may be shutting the whole works down if it’s receiving what it interprets as invalid or malformed commands.
I was trying the same thing years ago, with a mid-90s HDD, a dedicated power supply for it, and a couple different IDE-USB adapters, and never got it to work. The drive shutting down when USB adapter is plugged in sounds familiar.
This; works on Mull; there is no submit button, it just constantly refreshes the results and thus is slow AF from continuously juggling the data.