I know occasional problems are the most difficult to solve and if it is a rare issue (hw or sw) is really possibile that, in 15 years of work, you never deal with it. I have spent enought time on hardware and software like you and I'm having difficulty to solve this problem too.
Now, i'm going to re-explain the problem and to list all test and deductions I have done so you can see what you need and maybe together we can find a solution.
-) Laptop Configuration:
The laptop contain two different drive. The first one is a SSD where is install the operating system and applications and the second one is a HDD that is only a data container. (I bought this alienware 15 r3 only one month ago)
When I boot the laptop after all night or after short while, randomly, windows 10 doesn't detect the HDD (because i can't find it neither in disk management neither in device management and so neither in explorer tools too). When the problem show I solve rebooting the laptop a few times until hdd appear (because win 10 doesn't recognize the hdd at fist reboot usually).
If I forgot some tests I have done i'll update this post.
After this the only thing that I can think is a motherboard issue. Because if there is an alimentation problem on the motherboard that doesn't provide the right voltage to the hdd every time the laptop boot, I expect this problem.
I can prove that installing a linux distro on the HDD and booting every time from it. So, if the problem show up I'll be unable to boot from linux, and we can deduce is an alimentation issue.
I'm going to do this test and I'll update you. If you think something else please tell me.
I installed ubuntu 18 on the hard disk and every times I tried to boot from it (instead windows installed on ssd).
The problem show up again because I can't boot from ubuntu (see the image). Now I'm pretty sure is an hardware issue caused by sata connector or motherboard alimentation.
error: fail to write on sector 0x718d1018 on "hdo"
error: fail to read on sector 0x718d1018 on "hdo"
error: it's necessary to load the kernel before
push button to continue...
The problem was a motherboard issue for a manufactoring fault.
I solved calling Dell technical support and they proceeded to replace the motherboard and now it is working perfectly.