Constant "Reboot and Select proper Boot device" error even after system rebuild

The type of problem you report is a very difficult, if not impossible one to diagnose on the face of it.

It sounds as if it's a hardware-related problem and if so, the precise component causing the problem is virtually impossible to determine at the outset.

I assume you've tested the WD HDD with perhaps WD's Data Lifeguard program and no indication the drive is defective, right? BTW, does that drive also contain an OS or just used for storage/backup purposes?

Assuming a hardware-related problem one considers the possibility of a defective motherboard, but only as a possibility.

1. Assuming you have not as yet resolved the problem...just for starters, use the Microsoft Windows Memory Diagnostic tool or other memory diagnostic program you may be using.

2. While you're at it check the CPU with the Intel Processor Diagnostic Tool which you can download from Intel's website.

3. BTW, we're assuming the problem you've been experiencing occurs even if the ONLY DRIVE connected in the system is your SSD boot drive. If that's NOT the case, stop right here.

4. Assuming none of the above appear to be the problem...the only reasonable approach at this time is to "start over" as it were.

5. The basic approach would be along the following lines...
A. Get inside your desktop computer case (after disconnecting the A/C plug from your wall socket) and determine that the CPU, heat sink, memory modules & graphics card are all properly seated and all connections appear secure. Remove (disconnect) ALL peripheral devices including ALL HHD & SSD drive(s), optical drive(s), sound card, etc. Disconnect ALL storage devices, printers, and any other devices connected to the machine.

(As an aside, if it's not too onerous, it would be even better if your would remove the motherboard + components from the case and work with it outside of the case - perhaps on the cardboard case the MB was packaged in if you still have it or some other non-electric conducting surface, e.g., cardboard, wood, etc.)

B. In any event, all you'll be working with is your motherboard, processor, heat sink, RAM, graphics card, power supply, keyboard & mouse. Better yet, should your motherboard have onboard graphics capability, disconnect your graphics/video card from the system. Just ensure that following access of the BIOS/UEFI interface, if there is a BIOS setting for onboard graphics display that BIOS setting is enabled since that setting is frequently disabled as the default setting.

C. Reconnect your A/C cord and power on the system. What happens? Do you get a "normal" screen display? No error messages or strange notations from the system?

D. Can you access your BIOS/UEFI without any difficulty at this point and review the CMOS-BIOS settings? In particular, ensure that the SATA controller mode is set to AHCI (which should be the default setting). Check the hardware monitor/PC Health in the BIOS to determine that all temps are within normal range?

E. Assuming all is well at this point leave the system powered on for the next hour or so, checking to see if anything untoward shows up. Using your reset button, try powering down & up a few times to determine if there are any problems there.

F. Now you can install your graphics card in case it hasn't as yet been installed.

G. Along the way of the above steps if there's any indication of a faulty PSU, test that component with a PSU power tester (assuming you have one) or better yet, substitute a known non-defective PSU should one be available.

H. Should all appear well at this point this is an indication that there's nothing wrong with the basic components of your system. While not absolutely definitive this is so, it's a very strong indication that something else is amiss.

All this, of course, is to rule out at least to some degree) that the problem is not hardware-related insofar as the basic components currently installed in the PC.

Finally, install any BIOS update.

6. At this point install the SSD boot drive in your system and boot to the OS. Hopefully the system will boot & function problem-free. Work with the boot drive as the ONLY drive installed for some time to determine no problems exist.

7. Install your secondary HDD and similarly work with the system to determine all is well.

8. Then the other HDD.

I just don't see any other practical series of troubleshooting steps at this point but perhaps you have some other ideas. In any event keep us apprised of your progress (or lack of!). Good luck!