FATFS on SD-CARD writing problem

I use STM32F427 with True Studio as my IDE.

I need storing data on SD-CARD.

I use FATFS as my File System.

My problem is that most of the time the actual written data is not what is written - when trying to open the file I see squares only instead the characters I wrote (the number of squares is equal to the number of characters I wrote).

I must mention that sometimes its OK.

The configuration is like this:

&sharpdefine _USE_LFN 1 /* 0 to 3 */

&sharpdefine _CODE_PAGE 850

&sharpdefine _LFN_UNICODE 0 /* 0:ANSI/OEM or 1:Unicode */

using      

&sharpdefine _USE_LFN 0

didn't solve the problem.

Thanks