Why does my computer only sometimes recognize my hard disk drive?

This likely does not apply to you, but I have had this happen on drives approaching failure. Over the course of 100 boots the drive goes from being recognized 90% of the time to 50% of the time, to finally not working at all. So while it is likely what emgee or scoopdreams indicated, do back up your data ASAP.

"I am having exact same issue but I have a few more tools at my disposal to diagnose. Hopefully this can contribute to future diagnosis.

Maybe the power supply?

I have two hard disk drives hooked via SATA. The smaller/old drive contains the OS and started having issues after shortly after, but not right after, I swapped the PSU (500W, it failed after 6 years) for 450W at first, which was under powering my video card then for a 600W which worked like a charm. This event caused me to think it was a PSU issue as I don’t really believe in coincidences.

Maybe the old hard drive is at fault?

The other/newer hard disk drives was always detected, but contained no OS so couldn’t boot those times around. At first it only took a few retries to get it to detect and boot but every time it seemed to get worse. This coupled with the fact that the drive was also >6 years old, and the recent power issues led me to the same conclusion as Josh. So I quickly backed up everything as I didn¿t know how many boots I had left.

Maybe the motherboard’s SATA connector is defective?

Recently I found the only way to get the drive to detect was to swap SATA connector on the motherboard—I had already tried swapping cables around—and so far it boots if I swap them, but each new boot requires a swap. On top of this my DVD drive now isn’t being recognized sometimes. This leads me to believe it’s a SATA controller/mobo issue."

"So My laptop (inspiron 15 with an i3 core and recently updated windows 10) is having a very similar issue. It will attempt to boot and then tell me that the boot device is missing. the only consistent way I've found of making it boot up is to run it from the diagnostic menu, after a nearly successful diagnostic (cutting it part way through the extended part) i exit and reboot and it will reboot correctly some of the time. It (being the hard drive) sometimes makes really loud vibrating/ whirling sounds on an attempted and overall failed boot. the noises usually continue even through the attempted reboot after a failure.

After reading this thread I believe the problem might have been caused by my bad habit of rarely turning my computer off.... I often leave it on for a week at a time before it update reboots. I also often leave it rendering a 3d scene over multiple nights. I'm unsure of whether it is a mechanical issue (wiring) or a technical issue, but i hope this helps somebody get one last boot up to get files off the hard drive."