Prior to ISE design tools 11.5, if a design contained unused Block RAM sites and an attempt was made to readback the state of the Block RAM contents, a failure would occur. Failures will not occur on BRAM sites that are used in the design--they will readback correctly.
There is no workaround to this issue in the ISE design tools 11.4 and earlier. ISE design tools 11.5 and later software handle this correctly. ISE design tools 11.5 is currently available.