[SOLVED] Which letter is best for a smaller extra drive?

Solution
Anything after C.

A and B are legacy drive letters, windows treats them as floppy drives.
Meaning no Indexing, and maybe not TRIM if is an SSD.

D or higher.

"So largely the reason for starting the hard drive at C is for backwards compatibility. While the OS has abstracted data storage to some degree, it still treats A and B differently, in such a way that allows them to be removed from the system without altering the OS, caching them differently, and due to early viruses treating their boot sector with more caution than the hard drive’s boot sector. "
 
It will probably be just fine.

Me personally, I wouldn't.
I'm old school, and avoid A and B drive letters.
So far, I've had this setup like this for a few days now and all appears just fine, always wondered what should I do with this spare drive I had lying around since my other pc has no more space to fit another drive and I thought I was at max drives installed in this pc of mine, nobody wanted to buy it of me either, not until the other day when I decided to have a look for another sata power connection cable in my PSU's box, luckily I also had another sata data cable that had never been used.

Then I only needed to figure out how to mount yet another drive to my already fully loaded pc case, to my surprise I found that underneath the gpu and opposite the PSU there is more HDD mounts, mounting more drives does impair my ability to have a second gpu installed.