I am having a problem with mine as well. I recently bought a new 8gig SD card and was trying to change it out, buy I keep having to restart my phone to get it to connect to my pc.
I connect the phone via usb to my pc I get the dun dun sound and even the box that pops up shoing that the pc is reading the files but then nothing happens. So I unhook my phone restart it and try again. I finally get it hooked up to the phone where i can open the files on my phone. I noticed that if the phone goes into sleep mode it breaks the connection with the pc, and I have to repete the steps above all over again.
Once I have it connected I slide down the notifications bar and click on the mount option, and start to copy over files from my phone to my pc, but when I am done I slide down the bar again and select (uh can't remember exact wording, but something like turn off usb mass storage. However I get a warning message that says to make sure and unmount the SD card before doing so, but I don't see anywhere to unmount the SD card). So i have no choice but to just select turn off usb mass storage device.
This whole process seems to be a pain in the butt to just copy over simple pictures to my pc. I also noticed that there are two diffrent folders (that i have found so far) that seem to have pictures in it. The camera folder, and a folder called downloaded. Do I have to copy both those folders over to my pc so that I can later move them back to my phone with the new SD card?? If so does it matter if I put them all in the same folder or do i have to keep them seperate? Is there anything else I need to copy over to my pc before changing out the SD card?