To do it for both drives, just use /dev/hda and /dev/hdc for the second parameter.

It doesn't care that the partitions are mounted read-only, because it writes over the entire drive, including the partition table.

What it does is cause the disk to remap the bad sectors (the drive will only remap when writing).

Running out of space is expected: /dev/zero is an infinite-length stream of zeroes, and your disk has finite space.

(slightly drunk; being unnecessarily concise)