diff --git a/block.c b/block.c
index 65cf4dc..f837876 100644
--- a/block.c
+++ b/block.c
@@ -511,6 +511,7 @@ int bdrv_open(BlockDriverState *bs, const char *filename, int flags,
               BlockDriver *drv)
 {
     int ret;
+    int probed = 0;
 
     if (flags & BDRV_O_SNAPSHOT) {
         BlockDriverState *bs1;
@@ -571,6 +572,7 @@ int bdrv_open(BlockDriverState *bs, const char *filename, int flags,
     /* Find the right image format driver */
     if (!drv) {
         drv = find_image_format(filename);
+        probed = 1;
     }
 
     if (!drv) {
@@ -584,6 +586,8 @@ int bdrv_open(BlockDriverState *bs, const char *filename, int flags,
         goto unlink_and_fail;
     }
 
+    bs->probed = probed;
+
     /* If there is a backing file, use it */
     if ((flags & BDRV_O_NO_BACKING) == 0 && bs->backing_file[0] != '\0') {
         char backing_filename[PATH_MAX];
diff --git a/block/raw.c b/block/raw.c
index 4406b8c..92e41d8 100644
--- a/block/raw.c
+++ b/block/raw.c
@@ -9,15 +9,82 @@ static int raw_open(BlockDriverState *bs, int flags)
     return 0;
 }
 
+/* check for the user attempting to write something that looks like a
+   block format header to the beginning of the image and fail out.
+*/
+static int check_for_block_signature(BlockDriverState *bs, const uint8_t *buf)
+{
+    static const uint8_t signatures[][4] = {
+        { 'Q', 'F', 'I', 0xfb }, /* qcow/qcow2 */
+        { 'C', 'O', 'W', 'D' }, /* VMDK3 */
+        { 'V', 'M', 'D', 'K' }, /* VMDK4 */
+        { 'O', 'O', 'O', 'M' }, /* UML COW */
+        {}
+    };
+    int i;
+
+    for (i = 0; signatures[i][0] != 0; i++) {
+        if (memcmp(buf, signatures[i], 4) == 0) {
+            return 1;
+        }
+    }
+
+    return 0;
+}
+
+static int check_write_unsafe(BlockDriverState *bs, int64_t sector_num,
+                              const uint8_t *buf, int nb_sectors)
+{
+    /* assume that if the user specifies the format explicitly, then assume
+       that they will continue to do so and provide no safety net */
+    if (!bs->probed) {
+        return 0;
+    }
+
+    if (sector_num == 0 && nb_sectors > 0) {
+        return check_for_block_signature(bs, buf);
+    }
+
+    return 0;
+}
+
 static int raw_read(BlockDriverState *bs, int64_t sector_num,
                     uint8_t *buf, int nb_sectors)
 {
     return bdrv_read(bs->file, sector_num, buf, nb_sectors);
 }
 
+static int raw_write_scrubbed_bootsect(BlockDriverState *bs,
+                                       const uint8_t *buf)
+{
+    uint8_t bootsect[512];
+
+    /* scrub the dangerous signature */
+    memcpy(bootsect, buf, 512);
+    memset(bootsect, 0, 4);
+
+    return bdrv_write(bs->file, 0, bootsect, 1);
+}
+
 static int raw_write(BlockDriverState *bs, int64_t sector_num,
                      const uint8_t *buf, int nb_sectors)
 {
+    if (check_write_unsafe(bs, sector_num, buf, nb_sectors)) {
+        int ret;
+
+        ret = raw_write_scrubbed_bootsect(bs, buf);
+        if (ret < 0) {
+            return ret;
+        }
+
+        ret = bdrv_write(bs->file, 1, buf + 512, nb_sectors - 1);
+        if (ret < 0) {
+            return ret;
+        }
+
+        return ret + 512;
+    }
+
     return bdrv_write(bs->file, sector_num, buf, nb_sectors);
 }
 
@@ -28,10 +95,58 @@ static BlockDriverAIOCB *raw_aio_readv(BlockDriverState *bs,
     return bdrv_aio_readv(bs->file, sector_num, qiov, nb_sectors, cb, opaque);
 }
 
+typedef struct RawScrubberBounce
+{
+    BlockDriverCompletionFunc *cb;
+    void *opaque;
+    QEMUIOVector qiov;
+} RawScrubberBounce;
+
+static void raw_aio_writev_scrubbed(void *opaque, int ret)
+{
+    RawScrubberBounce *b = opaque;
+
+    if (ret < 0) {
+        b->cb(b->opaque, ret);
+    } else {
+        b->cb(b->opaque, ret + 512);
+    }
+
+    qemu_iovec_destroy(&b->qiov);
+    qemu_free(b);
+}
+
 static BlockDriverAIOCB *raw_aio_writev(BlockDriverState *bs,
     int64_t sector_num, QEMUIOVector *qiov, int nb_sectors,
     BlockDriverCompletionFunc *cb, void *opaque)
 {
+    if (check_write_unsafe(bs, sector_num, qiov->iov[0].iov_base, nb_sectors)) {
+        RawScrubberBounce *b;
+        int ret;
+
+        /* write the first sector using sync I/O */
+        ret = raw_write_scrubbed_bootsect(bs, qiov->iov[0].iov_base);
+        if (ret < 0) {
+            return NULL;
+        }
+
+        /* adjust request to be everything but first sector */
+
+        b = qemu_malloc(sizeof(*b));
+        b->cb = cb;
+        b->opaque = opaque;
+
+        qemu_iovec_init(&b->qiov, qiov->nalloc);
+        qemu_iovec_concat(&b->qiov, qiov, qiov->size);
+
+        b->qiov.size -= 512;
+        b->qiov.iov[0].iov_base += 512;
+        b->qiov.iov[0].iov_len -= 512;
+
+        return bdrv_aio_writev(bs->file, sector_num + 1, &b->qiov,
+                               nb_sectors - 1, raw_aio_writev_scrubbed, b);
+    }
+
     return bdrv_aio_writev(bs->file, sector_num, qiov, nb_sectors, cb, opaque);
 }
 
diff --git a/block_int.h b/block_int.h
index 877e1e5..96ff4cf 100644
--- a/block_int.h
+++ b/block_int.h
@@ -144,6 +144,7 @@ struct BlockDriverState {
     int encrypted; /* if true, the media is encrypted */
     int valid_key; /* if true, a valid encryption key has been set */
     int sg;        /* if true, the device is a /dev/sg* */
+    int probed;    /* if true, format was probed automatically */
     /* event callback when inserting/removing */
     void (*change_cb)(void *opaque);
     void *change_opaque;