this happened to me and nothing would work, not a different pc, not rebooting, not even reflashing the ROM. i Odin'd to stock EC05 with CWM3 and converted to EXT4, then flashed, and mass storage works now. i think i may have had a bad flash of EC05 originally - that was the last thing i did before SFR.
i also just went back to EB13 modem. i think this modem is better for my battery. signal strength wasn't very good on EC05
Just to clarify for those that replied, its not an issue with the pc, its the epic. I know because I tried it on more than one pc as well as my car stereo usb input. You plug it in, the epic brings up the choice of charging, mass storage, etc...and I choose mass storage. Next screen has the green android with the button that says turn on, press it and nothing happens. You can see the touchscreen registering the touch but nothing happens. Rebooting doesn't solve the problem. Unplugging and replugging in the usb doesn't fix it. Sometime later it just randomly starts working again. Annoying.