Doubt on using Array for 50000

Hi to all, The posting summery may be simple, but i am not able to fix the error.

Ok lets come to the problem. I am creating an Index
File "Test.txt" on Serial Flash using FILESystem which holds 50,000 datas each one is 2 bytes.

Another File is xx.txt which holds same 50,000 datas each size is 4 bytes. Using following function.

//MAX_USERS =50000

int File_Write_Index (unsigned int no_ofData)
{
        unsigned short count[MAX_USERS], i;
        FILE *fpt;

        fpt = fopen ("Test.txt","w");
    if (fpt == NULL)
    {
      printf ("File not found!. Write Failed\n");
          return 0;
    }

   else
   {
          for(i=0; i<MAX_USERS; i++)//MAX_USERS =50000
                count[i] = 0x0000 + i;

      fwrite (&count[0], 2, MAX_USERS, fpt);
          fclose (fpt);
          printf("\r\nWrite Succeed");
          return 1;
   }

}

and the second function for 4 bytes data same as first one where i change the size as 4.

fwrite (&count[0], 4, MAX_USERS, fpt);

Actually i made this function for 1000 MAX_USERS. But now i want to make it for 5000.

But Creating buffer for 50000 will increase the RAM size.
So i Created a line

unsigned long i=0;
unsigned char j;

for(k=0; k<MAX_USERS; )
{
j=(unsigned char *)&k;
fwrite (&j, 2, 1, fpt);
}

unsigned long j;

for(k=0x11111111; k<0x11111111+MAX_USERS; k++)
{
j=(unsigned char *)&k;
fwrite (&j, 2, 1, fpt);
}

But this one getting much time to read and write the files and even to search just for 1000 MAX_USERS.

   I am not able to go for more than 1000. The code does not print any errors. But its not working.

while debugging it goes to Somewhere which i dont know.

So i am here for your help and guidelines.
Please tell me how to create a line to move my 50000 no of 2 and 4 bytes of datas into my corresponding files? I am using LPC2388.