Interesting that in the manual for the F90BT, they would only state the following for SD card compatibility:
SD
Compatible physical format
..................................................... Version 2.00
Max capacity memory ........... 8 GB
File system .................................. FAT16, FAT32
Decoding format ......................MP3/WMA/AAC/WAVE/MPEG4
Maybe they didn't anticipate compatibility with larger cards? If you definitely have a 16 GB card working, then perhaps multiples may work, like 32, 64, etc.