I know this is an old post but for those of you searching the web for answers I thought I might offer my experiences. I have found that the most common solution to this problem is driver conflicts. Check your device manager while the faulty SD card is plugged into your system. First, look for the device that's listed with an exclamation point and uninstall it. AFTER doing this, unplug ANY removable drive devices (This includes printers with card readers) and restart your computer. After reboot, insert your faulty SD card back into your system. If it still fails to show up in 'My Computer', proceed to your Windows 'Control Panel'. Open 'Administrative Tools' and then load 'Computer Management'. Now select 'Disk Management'. Look for your removable device and if listed should show it without a drive letter assigned to it. Right click your removable device and choose the 'Change Drive Letter...' option. Now select a new driver letter for your SD card. I find it best to choose one higher than what it was trying to use before. This should help avoid a similar conflict from happening again. Hope this information helps some of you at the very least. Best of luck.