How to safely unmount a hard drive?

I've got an SSD running Windows 8 and two extra 500 GB SATA hard drives. While I'm working, the drives just randomly cut out—sometimes hard drive one drops, sometimes it's the other, or both—and they just vanish from My Computer

Check your power plan settings. If you dig into the advanced options, there's a setting for hard disk sleep timers. That might be your culprit...

Maybe you should take a look and see if you can find a way to disable that power saving feature altogether. Even if it stays turned on, though, the drive really shouldn't be dropping off like that once the system powers down.

I set the power plan to Balanced—turned off the display timeout and sleep mode entirely—and set "Turn off hard disk" to Never in the settings

Man, I dealt with this exact same headache about two years ago—I just couldn't figure out how to keep the drive from constantly spinning down or going inactive.

Check your BIOS settings—look for things like S1 or S3 sleep states, or anything labeled "Suspend to..." Try toggling those if you don't mind potentially messing with something else. It’s not a direct fix, but sometimes tweaking power states helps clear up those weird phantom issues...

Sometimes when I boot up Windows, it throws an error saying one drive is Capable but the command failed—but then the other drive doesn't even show the error before it just shuts down entirely. I’ve already swapped out the cables and tried different SATA ports on the motherboard... so, is this a software glitch or am I looking at a hardware failure? I tried to run an Acronis backup today, but the BIOS wouldn't even recognize the second drive where the backup is stored; then, after a restart, it suddenly shows up again.

It feels like there might be some intermittent electronic issue going on—like it’s working one second and failing the next. It could be the motherboard's disk controller, maybe even some faulty circuitry on the drives themselves.

I can't say for sure without testing it, though. Are all the connectors on the motherboard and the drives actually seated firmly? You might want to check if any solder joints have gone bad, too. It would probably be a good idea to run a scan on the drives using some bootable tools
(maybe Hirens, UltimateBootCD, or something similar).

I ran an HD Regenerator scan on the drives, and HD Sentinel is giving me the green light—one is at 92% and the other is at 98%. I’ve gone ahead and disconnected the specific drive that was acting up (the one throwing that "Capable but command failed" error), so I'm going to monitor how things settle over the next few days. It’s possible that error was what was causing the system to shut down, though I still don't quite get why the first drive was tripping out when it technically shows as okay.

I mean, you have to consider that they might just get phased out pretty soon if certain key sectors start leaning on them too heavily. Especially if things take a turn for the worse up top.