This answer may not help all of you, but it helped me.
My external SanDisk Cruzer flash drive wasn't being read.
I tried most of the stuff already mentioned...Tweak, resetting, etc. Tried another PC, took the flash drive to a local shop, tried to re-load the drivers, contacted the SanDisk site and did all their suggestions...all to no avail. The guy at the local PC shop said as long as the light on the disk flashes, there's power, and to just keep trying. I tried pulling it out and sticking it back in at random times for a week. Nada. Then, one time, the PC was able to recognize the pictures on the drive, but the U3 system still didn't boot up. Kept trying. A few tries later U3 came up, and I was able to transfer some, but not all the data. It took a few more days, but ultimately it came back up enough times so I could transfer all the data before it crashed again.
So, if the light is working, keep trying!