plexnowbot

I was trying to replace a hard drive in my Dell XPS 8500. The original hard drive was failing, but still working intermittently. It was running Windows 10, free upgraded from Windows 8.1, for which I never made the installation/recovery disk. So after a lot of research I figured out I could download the Windows 10 ISO. I did so, downloading it to a USB drive. I also installed a new internal hard drive, leaving the old hard drive in place. I used Windows 10 disk management, leaving it as one single partition, and assigned drive letter D. I then used the Windows 10 ISO on the USB drive to install Windows 10 to the new drive. I set the boot sequence to boot from the new drive.
I left things running overnight to get my massive dropbox synced to the new drive.
All seemed fine until this morning when I looked and saw a DOS-type screen again saying there was no boot device. I was able to reboot and I guess things seem OK for the moment, but there's no reason that I can think of that there should have been a boot failure from the new drive. So my question: Is it possible the old drive is still engaged in some way that would cause the whole system to fail? There's no 'disable' option in the device manager for this drive, making me think it's still necessary even though I intended to render it unnecessary by installing to the new drive. I guess I could go back into the computer and unplug the old drive and see what happens, but asking first seemed the prudent thing to do.
One other point, not sure if it's relevant: Even though I assigned the new drive letter D, the new drive is now (after Windows installation) showing up as drive letter C. But 'C' is definitely the new drive, based on drive size numbers.
I've spent three days getting to this point. I'm not a computer guy. Any insights appreciated.