How to verify that hard drive is filled with zeroes in Linux?

od will replace runs of the same thing with *, so you can easily use it to scan for nonzero bytes:

$ sudo od /dev/disk2 | head
0000000    000000  000000  000000  000000  000000  000000  000000  000000
*
234250000

You can combine this with either dd or pv to get a progress display while the scan is taking place:

$ sudo dd if=/dev/disk2 bs=1M status=progress | od | head

# or:

$ sudo pv /dev/disk2 | od | head

I've written a short C++ program to do so, source available here.

To build it:

wget -O iszero.cpp https://gist.github.com/BobVul/5070989/raw/2aba8075f8ccd7eb72a718be040bb6204f70404a/iszero.cpp
g++ -o iszero iszero.cpp

To run it:

dd if=/dev/sdX 2>/dev/null | ./iszero

It will output the position and value of any nonzero bytes. You can redirect this output to a file with >, e.g.:

dd if=/dev/sdX 2>/dev/null | ./iszero >nonzerochars.txt

You might want to try changing BUFFER_SIZE for better efficiency. I'm not sure what an optimum value might be. Note that this also affects how often it prints progress, which will affect speed somewhat (printing output to the console is slow). Add 2>/dev/null to get rid of progress output.

I am aware this is not using standard bash, nor even builtins, but it should not require any extra privileges. @Hennes' solution is still faster (I haven't really optimised anything - this is the naïve solution); however, this little program can give you a better idea of just how many bytes your wiper has missed, and in what location. If you disable the progress output, it'll still be faster than most consumer hard drives can read (>150 MB/s), so that's not a big issue.

A faster version with less verbose output is available here. However, it is still a little slower than @Hennes' solution. This one, however, will quit on the first nonzero character it encounters so it is potentially much faster if there's a nonzero near the beginning of the stream.

---

Adding source to post to keep answer better self-contained:

#include <cstdio>

#define BUFFER_SIZE 1024

int main() {
    FILE* file = stdin;
    char buffer[BUFFER_SIZE];
    long long bytes_read = 0;
    long long progress = 0;
    long long nonzero = 0;

    while (bytes_read = fread(buffer, 1, BUFFER_SIZE, file)) {
        for (long long i = 0; i < bytes_read; i++) {
            progress++;
            if (buffer[i] != 0) {
                nonzero++;
                printf("%lld: %x\n", progress, buffer[i]);
            }
        }
        fprintf(stderr, "%lld bytes processed\r", progress);
    }

    fprintf(stderr, "\n");

    int error = 0;
    if (error = ferror(file)) {
        fprintf(stderr, "Error reading file, code: %d\n", error);
        return -1;
    }

    printf("%lld nonzero characters encountered.\n", nonzero);
    return nonzero;
}

Expanding on Gordon's answer, pv provides an indication of how far along the process is:

$ sudo pv -tpreb /dev/sda | od | head
0000000 000000 000000 000000 000000 000000 000000 000000 000000
*
9.76GiB 0:06:30 [25.3MiB/s] [=================>               ] 59% ETA 0:04:56