dd doesn't overwrite the disk

As said in the comments, the sdcard was with badblocks.

The solution I proposed was to run:

badblocks -t 0x0000 -sw /dev/sdc

CAUTION: this is data destructive like dd if=/dev/zero.

And the user received something like:

7234624 done, 39:10 elapsed. (0/0/2417408 errors)

Showing the sdcard was damaged.

The sdcard was replaced and the problem was solved.