Your PC could stop detecting your Blackberry's memory card when connected for a number of reasons. There are however three things you could do to solve this problem.
1.Go to options > memory (or storage) then set Mass storage support to 'ON' and set Auto Enable Mass Storage Mode When Connected to 'PROMPT'.
2. Connect the Blackberry to the PC then launch Blackberry Desktop Manager. The SD Memory card will be forced to be detected.
3. Try a different cord because the current cord might not be making adequate contact for data transfer.
