Upload & Sell: On
In my experience if I use one multi-slot reader to read two cards simultaneously each slot gets a drive letter assigned to it while in use, but not if the slot's empty. I don't believe your quest for speed and your desire to limit the number of drives are congruent.
One speedy multi-card reader or individual readers, one per card, will give you the same number of drives, won't it?
Actually no, if you don't assign a drive letter then it only shows up for the slot you insert the card in, however if you are already using that drive letter e.g H:, like i do since my Home drive is H and i have some other network locations mapped to my NAS for music/movies etc that may conflict with the drive letter the media reader wants to use.
I had to assign the drive letters to the card reader as it wanted to use some of the same drive letters that i had assigned to my home drive (H: or G so forth), so on my computer all 6-8 drive letters for the media reader show up in my windows explorer.