diff --git a/mmc.c b/mmc.c
index 07f219e..a0e41e1 100644
--- a/mmc.c
+++ b/mmc.c
@@ -99,6 +99,15 @@
 #define STATE_ADDR_ERR      (1 << 5)
 #define STATE_PARAM_ERR     (1 << 6)
 
+// data response token for write operations
+// format: xxx0sss1 (8 bits), where bits [3:1] (sss) indicate status:
+// - sss=010: Data accepted    → full token masked = 0x05 (xxx00101)
+// - sss=101: CRC error        → full token masked = 0x0B (xxx01011)
+// - sss=110: Write error      → full token masked = 0x0D (xxx01101)
+// mask extracts bits [4:0] to include the fixed bit 0 (always 1) and bit 4 (always 0)
+#define DATA_RESPONSE_MASK     0x1F
+#define DATA_RESPONSE_ACCEPTED 0x05
+
 #define TIMEOUT 0x7fff
 
 // private helper functions
@@ -332,7 +341,16 @@ void drive_write_block(uint32_t blkno, const uint8_t *data) {
   spi_send_byte(0xff);
   spi_send_byte(0xff);
 
-  // wait while card is busy
+  // read and verify data response token
+  // the card responds with a token indicating if data was accepted
+  uint8_t response = spi_rec_byte();
+  if ((response & DATA_RESPONSE_MASK) != DATA_RESPONSE_ACCEPTED) {
+    // data rejected: either CRC error (0x0B) or write error (0x0D)
+    tfs_last_error = TFS_ERR_IO;
+    return;
+  }
+
+  // wait while card is busy (card pulls line low during programming)
   if (!wait_byte(0xff)) {
     tfs_last_error = TFS_ERR_IO;
     return;