I ran across this post and thought I would add a comment to any future google searches.
I image Transcend IDE flash modules via Sabrent USB interface and encountered this problem.
When inserting the USB, the normal sound can be heard of insertion, but the device never appears in My Computer and can not be accessed. I tested this on WinXP and Win7. The device has bootable information, and in fact boots perfectly in another environment, so the device still works, just damaged in such a way as Windows will not recognize it.

Solution- Use Winimage.com to rebuild the module or CF card. If you want to keep photo's or data on your current CF card, you will need to have a good recognizable CF card on hand that your PC will recognize. Start Winimage with a good CF card. Select the device (Disk->Use removable disk "x"). Remove the CF card and insert your unrecognizable unit. Now select Disk->Creating Virtual Hard Disk Image from Physical Drive. Select the Disk (in my case a 1Gb CF card was Disk2 : 990,864 KB - Generic Storeage Device 9339). Give it a name and store on your pc as *.vhd file. Now select Disk->Restore Virtual Hard Disk Image from Physical Drive. If all went correctly, you can remove and install the CF card and Windows will once again see your device, with all your data as well.