SD card multiple read

Dear Tricky, you can name it a gap in knowledge , may be , because I had implemented many circuits before as SDHC, wavelet and VGA... I show the error in this code because it was the same when I used the SRAM in code below .

IF (wr_done='0') THEN

wr_enable := clk;

test <= D(7 DOWNTO 0);

A <= std_logic_vector(to_unsigned(i,18));

ELSE

test <= D(7 DOWNTO 0);

A <= std_logic_vector(to_unsigned(vac,18));

wr_enable := '1';

END IF;

nWE <= wr_enable;

wdone_test <= wr_enable;

--Bidirectional bus:--------------

IF (wr_enable='0') THEN

D <= "00000000" & myram(i);

ELSE

D <= (OTHERS => 'Z');

END IF;

END PROCESS;

END ARCHITECTURE;

here when I replaced myram with SD data I got the same error. And the code above worked well with VGA and images are shown correctly and the ram is created. If I had a problem with basics it wouldn't run eventually. However if you think it is a basic question , I wish you guide me to what to read to pass it.

Thank you

With my best regards.