1. what make, size and file formats are your SD cards.
2. Did you leave it on and playing music overnight while it's hooked to your phone charger? Did you try to change it directly from a PC or laptop. Your phone charger may not have enough current and doesn't charge the DX90 battery fast enough.
3. Answer 2 first.
1. I tried two microSD cards, Toshiba 32Gb Sandisk 64Gb, both were formatted Fat32
2. a. I did but it did not work and kept being charged
b. I tried but it was too slow
c. I charged it using an LG phone charger, rated at 2.1A, and a Samsung phone charger rated at 740mA
Worst thing of all, the process seemed cause a short circuit in my mains supply.