one thought.....Hubby suggests going to Lexar's website to see if they have a downloadable driver so your computer can communicate with the card.
another thought....Did you format the card before using it?
third thought...if nothing else works, get a (hubby says $20) card reader from Lexar.
Last minute "wait a minute"....hubby says 8GB is HDSC, not a normal SD card, and the normal "reader" in your computer probably can't communicate with it. In that case, you'd definitely need to get a card reader that reads high capacity cards.
But he still suggests checking the Lexar site in case they have a driver that works for your situation.