Sudden "Reboot and Select proper Boot device"

Hello all,

I custom built my machine in the summer of 2013, and it's worked flawlessly for 5 years (see specs below). Upgraded the video card once a couple of years ago, but all other components are original.

The problem first showed up this afternoon. The system had been powered up for about 5 days and I had been working for most of this morning with no apparent issues. Then I tried to launch a game in Steam and it notified me that the game's files could not be located. Restarted, resulting in multiple failed boots with the above message.

I have two main drives: The OS is on the SSD, and the majority of my files (including the data for all Steam games) are on the HDD.

This one is confusing me. The boot failures made me think that my SSD had died (it is almost 5 years old, after all). But the fact that everything was working fine until I tried to launch a game also makes it look like the HDD could be involved. The kicker is that when I enter the BIOS on startup both drives are still being detected and are showing the approximately correct quantity of data present. HOWEVER, the detection of the HDD by the BIOS appears to be intermittent, with the drive sometimes showing up and sometimes not. But again, if this is the issue, why would I have a boot failure, since the OS is on the SSD, which always shows up?

Apologies for the long-windedness, but I wanted to provide as much helpful information as possible. Any help and/or suggestions would be much appreciated.

Motherboard: Asus P8H77-I
Processor: Intel Core i5 3570K 3.4 Ghz
Video Card: Sapphire Radeon RX 480 8 GB
Memory: Crucial Ballistix Sport 16 GB (2 x 8 GB) DDR3 1600 Mhz
Hard Drive: Seagate Barracuda 2 TB
Solid State Drive: Samsung 840 120 GB
Optical Drive: Samsung SH-224BB
Operating System: Windows 7 Home Premium 64-bit

Welcome to the forum. Check all connections to the drives are ok. Then run disk check on both drives it can be 2nd drive failing and this can confuse the boot up as the BIOS can be retrying to detect drive locking up the BIOS

Also, include a screen shot of your Disk Manager. Sometimes during the installation, Windows will put the boot files on the first disk it sees and that may have been the standard hard drive.

Update: It looks like the HDD was bad, which caused decidedly strange symptoms. The operating system was on the SSD, so I can't figure out for the life of me why that would cause boot failures. Even when the HDD was fully disconnected from both power and data the system still couldn't boot successfully. Is there any way anybody can think of that an HDD failure could cause the corruption of the OS on a separate SSD?

Anyway, I disconnected the HDD and did a clean reinstall on the SSD, and it seems like everything's getting back to normal. Just have to get a new drive. Weird. Thanks for the suggestions, everybody.

As explained the BIOS trys to read the disk on start to get details if it's faulty it can keep trying and get locked up and may even get confused with the other drive