Strange behaviour FLASH FS

Hello!
We use KEIL 4.53 and FLASH FS library.

When i write next code in my function, after compile and start debug session i can't goto main();

FILE *fd;
fd = fopen ("S:\\1.dat","w");

But when i use :
finit ("S:");
fcheck ("S:");
ffree("S:");
fformat("S:");

there is no problems to use them.

Interface: SSP0, memory AT45DB161D,SSP0 Frequency 1Mhz, CPU LPC17778(Cortex M3)

HEAP SIZE: 0x1000
STACK SIZE: 0x1000

I don't understand what's wrong???
Please help!