I experianced the issue of my sd unmounting several times, have reformated dozens of sd cards, factory reset several times, rooted, unrooted, and flashed/reflashed Roms. Almost bricked my phone twice, but found a way to get the sd card back.
As long you are useing clockworkMod anyway. If you lose mount on the sd card, reboot into recovery. Go to the "advanced" options in recovery. Select mount USB. Connect to pc, may take several times plugging and unplugging, till you see the drive come up on your pc (as F: or E: or whatever) Once you see that, back out of the mount usb screen in recovery. Go to an option like install zip from SD, just to varify sd mounted. You should see folders and perhaps files available, back out of that option(unless you want to flash a rom) and select to reboot your device.
Once rebooted your sd card should still be mounted. I hope this works for you, as it has me.
Linear Mode
