It is a GW3DS bug. The exFAT partition info contains the cluster count, unlike FAT16/32 partitions. The first data cluster starts at number 2. Number 0 and 1 are reserved. If the number of clusters is "n" then the number of the last cluster is "n+2". There is probably a check somewhere in the GW3DS code to see if a cluster number is valid where they compare the cluster number against the cluster count, without adding/subtracting 2. My program "releases" those two last clusters if they are in use by a file and then creates a small file named "_GW3DS_DO_NOT_DELETE.BIN" in the root directory which will occupy the last two clusters, to prevent the fragmentation error from reoccurring.