Two possibilities worth checking;
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more likely
There are lots of bugs in ADE2.0; it appears that Adobe put it out with almost no product test.
One of these is that it doesn't recognize many devices that it ought to.
The solution is to replace it 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
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less likely
Some devices (Sony ones in particular) need special drivers before they will be recognised by ADE;
even though they are seen as mounted devices without the special driver.
In the Sony case, the driver is installed with the Sony Reader software, and will work even if you never run the Sony Reader software itself.
I don't know if the Nook has a similar irritation, but it is worth checking the instructions that came with it.