the default cluster size cant be 4k...
FAT32 supports 512, 1024, 2048, 4096, 8192, 16K, 32K, 64K, (128K, 256K for sector size > 512 bytes).
I did a format with windows using default and it worked if i have about 6 games in the directory where UMK is.
But if i add all 60 games in that...