Simple, you have to MOUNT the card, not just plug the USB to the phone and computer. When that drive says "Removable Disk" and you click on it it pops up saying "Please insert a disk into drive E:" it means mount the card. The drive tells you the card name when its mounted, and does not give you this message when mounted.
When you plug your phone to the computer, it shows up as a drive, but the SD card is not accessible until you mount the card.
On 2.1 :
-Pull down the notification bar
-Select the USB icon (it may already say Charge Only)
-Now choose Disk Drive
In your my computer, you will notice the drive changes and if you double click on it now its accessible.