Depends on how the reader works. Some readers interact with windows like a floppy disk or CD where the drive holds a letter and if you click on the empty drive windows asks you to insert a disk. Some readers act more like a USB thumb drive. Where the letter is not assigned until the mem stick is plugged in.
If your reader is like the first mentioned you can go to control panel
unchceck hide empty drives in the computer folder.
Then go back to disk managment and change drive letter as normal.
If the reader works like the second option you can change the drive leter in disk manager only when a mem stick or sd card is plugged in. But keep in mind its like a USB thumb drive, the new drive letter will be retained by that particular Sd card and if you plug in another you will have to do it again.
It works like a floppy/CD. I went into folder options and found show empty drives already unchecked.
It turns out to be one of my "Dooh" moments. In the upper view pane, it shows only hard drive and only them in the bottom view pane, until I expanded the bottom pane with revealed the card reader drives! They were there, but the pane was too narrow to show them.
How embarrassing. (But I do like it when it turns out to be something simple.)