It could be a lot of things. Just a few are:
- Bad / Flakey Hard-Disk-Drive
- Bad Cabling
- HDD overheating since it's in small closed-up laptop
- Outdated or buggy CMOS BIOS Firmware
- Bad Motherboard
- Corrupt software or OS.
 
Since you have the luxury of it being a secondary HDD, you might want to remove it, install in external USB enclosure or tester and see if it works that way (maybe even on another computer).
 
Are you sure you don't have a friend or family member that can help? If not, you might have to seek professional assistance. Troubleshooting is not hard, but can be confusing and time-consuming if you don't know what you are doing. You can also easily make things worse (ie kill the whole computer). 

I experienced a similar issue when I updated to windows 10, so I'm just going to throw this out there. I checked my bios and found that I needed to set the other drive(s) to active (or whatever) in order for my OS to "see" them. I am not an expert and I am not saying this is the fix for you. Or that you should get into the bios if you are not comfortable with doing so, I'm just saying I had a similar issue when I moved to windows 10 and that is where I found the answer. GL

Hi ‌,

If you still need help with this, send me private message with the service tag.

Hello, my D drive also disappeared, can you help me?. Thanks

Hi ‌, 

Send me a private message with the service tag. 

hello, how can I  send you a private message?

Just hover the cursor over my name and click on message. 

sp4ztik wrote:

1. I also have tried using dell diagnostics to help and all I receive is "your product is out of warranty" message.

2. I have noticed, that my D drive will "appear" randomly when I restart the computer or just whenever it awoken from sleep mode.

1. The ePSA test from BIOS doesn't do any checks like that.

2. I'm guessing it's a conventional spinning hard-drive. I would first power-down computer and try "re-seating" it's electrical connection to the computer. See PDF Service/Owners manual.

Then, if still problems it might be defective. One common problem is they get a bit too hot and shut-themselves down for a while. They aren't expensive or hard to replace. SSDs are also cheaper these days. A bit less space, but use less power and IMO, can be more durable and dependable.