First of all, I have an external hard drive. What's strange is that it doesn't spin up with I turn the computer on, BUT it DOES spin up when the computer is off. And it's detected by Windows, but it can't be initialized so I can use it.
And my second internal hdd isn't recognized by the BIOS, but my boot hdd is.
it must have some built in power saving thing to disable spin up when not properly detected. if you have a usb hub that should make it stay on permanently. otherwise repluging it should make it turn on. perhaps you can disable usb power saving inside of your computers power options to make it stay on. your internal HD, is it sata or IDE? if sata then you may not have it pluged in all the way. if its IDE it may have the wrong jumper settings.
1. Your USB suggestion worked! I went to Power Options > Edit plan settings > Change advanced power settings > USB settings > USB selective suspend setting. Changed it from "enabled" to "disabled," plugged my external hdd in, and immediately, the led came on, it started to spin, and was then detected and Autoplay came up! I am so happy!
2. As for the internal hdd, I haven't opened up my computer for the past day or so, so I can't tell you if it's not plugged in all the way or not. But, I formatted the hdd and deleted the volume AFTER it wasn't detected by the BIOS. I have yet to add a volume to the not-allocated space. So I'll probably do that with gparted on my old computer and see if that works.
Update: So I formatted my second internal hdd with gparted on my old computer. Plugged it into my new computer, and was immediately detected in the BIOS. Everything is essentially "back to normal before I built a new computer."