PhoenixEvo

It's terrible to lose data. Having said that, I don't follow what your situation is. I have never had any data lost from adding a HDD to DrivePool. 
Is it your Drive D that you believe was formatted? That's the only drive that appears to have no data.
On the top left corner of the Pie Chart DrivePool main page, you have 3 messages logged. You might want to click on that small pie chart and read what DrivePool has logged. Sometimes that gives you clues.
Is there a reason you added your Drive C to your DrivePool? I know it's possible to add Drive C to DrivePool, but I personally never use my Drive C for any pools. I try to keep that Drive C from outside storage programs.
As far as trusting DrivePool, well, it works better for me than my old hardware RAID setups, or when I used Windows Storage Spaces. I had massive data loses with a single HDD failure in both those other systems. In fact, even though I had triple redundancy on my Windows Storage Spaces, I had a single HDD failure and it destroyed my entire Storage Space volume. That was not supposed to happen.
I switched over to DrivePool a couple years ago. I have had HDD failures using DrivePool, but it only affects the data on the failed HDD. Many times, I was able to physically remove the drive, plug it into a desktop caddy, and transfer almost all the data off that failing HDD. 
DrivePool is not a backup plan, but you can turn on duplication for the entire pool, or specific folders. In theory, that should allow you to rebuild your data faster when you add a new drive to the pool after a HDD failure. I find that useful for some data. Mostly, my DrivePool is used as my Home Media Server and I have all my data backed up on HDDs sitting in my closet. I mostly trust DrivePool to handle my server data, but like anything with computers, you really need to have a good backup plan to ensure no data loss if you have HDD failures. After running DrivePool for a couple of years, I only have single copies of media data on my DrivePool Home Media Server. If I lose a pool HDD, then I'll rebuild from my backup HDDs if still needed.