@Kas: I'm not 100% sure why this happens, but I
think this is because of a combination of the following;
(1) the card reader is not a 'real' drive, but is a USB device masquerading as a drive and
(2) that the virtual drive is seen as a SCSI device which is automatically put first in the order of CD/DVD drives. Thus causing the other devices [after C] to automatically re-order themselves.
If this is true then the drivers for the card reader are flawed in that they appear to be bound to a specific letter rather than allowing a dynamic drive letter allocation like it should be.
My suggestion is to disconnect the USB card reader, install the
CloneCD virtual drive [should you wish to use it] then use Device Manager to re-assign the virtual drive to a higher letter. I tend to use V: [for Virtual] but that is just my choice. The re-attach the card reader and it should come back as G: