For those who are having difficulty with proper scanning with their DX90, perhaps my experience will prove helpful....
I removed the micro SD card from my DX50 where it was playing flawlessly and inserted it into my DX90. (all of my music is on the sd card, none on the internal memory) The option for scanning appeared and I selected "scan". When finished, it first appeared that my music was present but after listening to the shuffle mode for awhile, it was obvious that many of the same tunes and/or artists were being repeated. I then checked in "Albums" and found only a small number of albums and artists showing. Likewise, in "All Music", there was only a small number of files showing of what I knew was actually present. Many of the albums showed "null".
I then, leaving the card in place, accomplished a "wipe and factory reset". This was done by holding both the volume up and power button together, not by selecting from the settings menu. When this procedure was completed, once again the scan option appeared and once again I selected "Scan". This scan took about 4 or 5 minutes. (my 64gb card is nearly full) Following the completion of this scan, all of my music, albums and artists appeared exactly as in my DX50. All of my albums are by folder, one album per folder. I drag and drop from my computer and do not use itunes or any sort of tagging program.
Since then, my DX90 has performed flawlessly.