Make sure every thing else is marked "On" in the memory options. Media card support, MTP, and Mass storage support mode. Save those settings and then perform a battery pull. Hope that helps.
Same problem with my 8330. Nothing works, I tried it all. I wish there was a way to access the SD card without using the Blackberry Desktop, I think that's the problem! I can access it through an E-Book software and put books on the card so I know the connection is good but it has no previsions for sending pics or video.
When you connect your BB to your computer via USB cable have you tried to click on your PC computer icon and see if it is listed as an external drive? That is how I move media on and off my BB. I do not use DM.