[jasn]

(again) Thanks Kovid,

This was a key phrase from your referenced section of the manual for me;

Note that if you are using the user defined plugin for a device normally detected by a builtin calibre plugin, you must disable the builtin plugin first, so that your user defined plugin is used instead.

Using these instructions, I was finally able to configure Calibre to achieve my goal. For anyone searching, who finds this thread, this is what I did:

1) Under Preferences -> Plugins -> Device Plugins, disabled all Device Plugins except for the one titled, User Defined. (I realize that it isn't necessary to disable all plugins, but I really only need one for my purposes)

2) Customized the User Defined plugin by selecting all supported reader formats for my device, and then I entered in the 3 USB ID numbers in hex, for my Nook Color in the fields marked, USB Vendor ID, USB Product ID, and USB Revision ID. While Calibre does have a feature to extract these numbers using the option at

Preferences -> Miscellaneous -> Get information to setup the user defined device

it was coming up blank for me. Instead I chose the

Preferences -> Miscellaneous -> Debug device detection

which output information for all of the USB devices connected to my computer. In the line that contained

[hex number 1], [hex number 2], [hex number 3],'B&N', 'NookColor',...

I was able to get the information necessary to configure the User Defined device plugin. Hex number 1 is the USB Vendor ID, hex number 2 the USB Product ID, and hex number 3 is the USB Revision ID. After I applied the changes, my Calibre library began recognizing all the books in my Nook Color sdcard's /books/ folder, so I know that my communications are now working correctly.

I then used Calibre to delete all books on my Nook, and then used Calibre to send just one, to my device's main memory. However, even though I had configured the User Defined device plugin Main Memory Directory field with, books, Calibre copied the book to the root directory of the sdcard. So borrowing from my first test, I deleted the User Defined device plugin Directory field, and added books/ immediately preceding the {author_sort}/{title} - {authors} string in the device plugin customization. Now when I send books from Calibre to my rooted Nook Color, they end up being copied to a subdirectory of the root directory, books, on my sdcard. Which is what I was after.

Thanks again..