Question System forgets boot device priority

[jamok99]

I understand that there is no one definitive answer to my question, but all suggestions are appreciated - TIA. I'm running Windows 7 sp2 on an MSI Z87m motherboard with an Intel 4590 CPU. A couple of weeks ago, my system started failing to boot, with a black screen telling me to put in system repair (or maybe it was emergency disk) and press any key. I do have an emergency disk I made,
but putting it in and pressing any key did nothing. The failure to boot also made my keyboard (A logitech G710 gaming keyboard) and my mouse inoperative, so I wasn't able to get into bios to check the boot priority list or other possible problems. However, my system has a PS2 port, and plugging in an old PS2 keyboard, and restarting the system, let me push F11 until it let me into bios. What I found was this: My system has 3 drives - the boot drive is a Crucial SSD M500, as well as secondary drives - a Seagate 1tb mechanical drive, and another Crucial 500 SSD, but that latter being a different model - BX300 I believe. The system seems to have been trying to use the BX300 to boot, which of course would result in failure. When I correct it to boot from the Crucial SSD M500, all is good again. However, this problem recurs the majority of the times I boot my system, and have to go through the steps again, using the PS2 keyboard to get into bios.

So, (sorry to be long winded, but detail matter), I'm wondering if I can get some opinions on what's going wrong and how to fix it. My guesses are these:

1. Bad data cable on the boot drive?
2. CMOS battery starting to wink out?
3. Given the system was built about 6 years ago, perhaps the cpu paste is wearing out (I think that is unlikely)
4. Some time before this started happening, I was running Windows 7 sp1. I decided to update to all the available Win 7 patches (there were probably like 100), and the machine

seemed fine at first, although boot times were noticeably longer. Perhaps one of the patches is screwing up my system?
5. The most obvious choice, which is unlikely, is that my boot drive is bad/corrupt. I know this because I have another SSD configured as a boot drive running Windows 10. Even with
the Windows 10 drive, the problem happens the majority of the time upon attempted boot-up.

TIA for the help!

 

[USAFRet]

#2, followed by #5.

 

[jamok99]

Thanks for responding. But I'm perplexed as to why you think #5 is likely. Two different SSD drives, one running Win 7 and the other Win 10 have been set up for this configuration, yet when swapped into the system as the boot drive, they both exhibit the problem of being unable to remember the correct boot sequence. I'd think that'd rule out the problem being centered around the boot drive, no?