You stated that "It seems your Aurender is convinced that your USB drive is the same (in terms of contents) as the internal HDD. The external USB drive has 1700 albums mostly AIFF. The internal drive has 946 albums, all m4a (ALAC) files".
I think your Aurender is getting confused between the USB drive and the internal HDD. I believe the Aurender always uses the HDD and needs to be told to use the USB drive. After you insert the USB drive, go into settings and see if you can direct the Aurender to play albums from the USB drive. I have never done this before and am guessing. Please review and let me know. I will do additional research and let you know.
I think your Aurender is getting confused between the USB drive and the internal HDD. I believe the Aurender always uses the HDD and needs to be told to use the USB drive. After you insert the USB drive, go into settings and see if you can direct the Aurender to play albums from the USB drive. I have never done this before and am guessing. Please review and let me know. I will do additional research and let you know.