This may or may not be your problem but is looks like it. I found various "memory cards" work on some systems and not others. Usually, when installed in laptop/camera they work great but not in some multi-function type reader or ext. reader. I can only summise that basic readers are cheaper than others. Clean the cards plus the reader with cleaning pads for best *contact* and take it from there. the fingers/pins on some readers just aren't "stiff and or tight" enough for more robust contact and are easily defeated. If you can wiggle the cards up to the contact side of the reader and see if the system then sees it. In ther words force a better contact. Thsi solution excludes any s/w glitches, etc. but has been proven to help in many cases, but a replacement reader usually is the true fix.

tada -----Willy Happy