Thanks @iceman1001 for catching my oops, and fixing it :)
        if (MF_DBGLEVEL >= MF_DBG_EXTENDED) Dbprintf("Blocks read %d", countblocks);\r
 \r
        countblocks *= 4;\r
-       cmd_send(CMD_ACK, 1, countblocks, countblocks, 0, 0);\r
+       cmd_send(CMD_ACK, 1, countblocks, BigBuf_max_traceLen(), 0, 0);\r
        FpgaWriteConfWord(FPGA_MAJOR_MODE_OFF);\r
        LEDsoff();\r
 }\r
 
                return 1;
        }
 
+       uint32_t startindex = resp.arg[2];
        uint32_t bufferSize = resp.arg[1];
        if (bufferSize > sizeof(data)) {
                PrintAndLog("Data exceeded Buffer size!");
                bufferSize = sizeof(data);
        }
-       GetFromBigBuf(data, bufferSize, 0);
+       GetFromBigBuf(data, bufferSize, startindex);
        WaitForResponse(CMD_ACK,NULL);
 
        Pages = bufferSize/4;