From Jeremy::::

NOTE:
Using SDFSHC32 Revision D,
IF (Your PIC is running at 40Mhz) AND (You are using the DEFINE FS_SPEED) AND (Maybe your PCB and/or wiring isn't quite 'up-to-snuff') THEN
You may have initialization/writing problems which WILL corrupt your SD card
ENDIF

Workaround: Don't use "DEFINE FS_SPEED".

The problem is either a R/M/W issue or SD card timing problems at higher PIC speeds.
END-NOTE