It may be that you need special driver software (really irritating).
Sony eReaders do. Without the driver they show up as a Windows mounted drive, but won't show up in ADE.
The driver is installed with Sony Reader software supplied with the eReader;
you don't need to run the Sony Reader software, just install it to make sure the driver is installed.
I don't know if the same is true of your Pandigital. Check the manual that came with it just in case.
There wasn't any such nonsense with a Bookeen Opus I had.
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There are lots of bugs in ADE2.0. One of these is that it is stoppy about recognizing some eReaders.
Try replacing ADE2.0 with the older but more reliable v1.7.2.
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
http://helpx.adobe.com/digital-editions/kb/cant-install-digital-editio ns.html
The forum software is sometimes corrupting the link above. There shouldn't be a blank in 'editio ns.html'. The following redirects to the same page: http://tinyurl.com/diged172