Sorry, I'm going to answer by questions/comments so I am more clear exactly what is happening ...
You never read a .acsm book directly.
The .acsm file is just a token allowing download of a DRM encrypted .epub file.
That open of the .acsm and download of the .epub is typically performed by ADE on PC/Mac,
but can sometimes be done directly on a device.
It is the .epub file that is the real eBook.
Once the .acsm file has been used to enable the download it does not have any real value,
though it can be used to repeat the download of the .epub.
The same Adobe ID must be in use for the second download as was active for the first download.
As far a I am aware, ADE always transfers the .epub to the device, never the .acsm file.
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When you say
I open the book by opening the Nook device, going to the appropriate directory, and clicking on the file number (all the books are listed by file numbers, not titles).
do you mean you open the book on the Mac reading from the Nook file system,
or do you mean you open the book on the Nook while it is still connected to the Mac?
Similarly
But when I disconnect the Nook form the Mac only B&N books are shown by title, not the one I downloaded.
is this on the Nook that only B&N books are shown? Or on the Mac now that the Nook is disconnected?