Keep trying to copy a file until success.

freadit.php

<?php
$path = '/Volumes/drive/path.dat';

$write_to = '/tmp/output.dat';
$fh_out = fopen($write_to, 'w+');

clearstatcache();

while (!file_exists($path)) {
    sleep(1);
}
$stat = stat($path);

$size = $stat['size'];
$seek = 0;

print_r($stat);

while ($seek < $size) {
    sleep(1);

    $fh = fopen($path, 'r');
    if ($fh === false) {
        printf("failed to open file\n");
        continue;
    }
    printf("opened file\n");

    $seeked = fseek($fh, $seek, SEEK_SET);
    if ($seeked === -1) {
        printf("failed to seek\n");
        @fclose($fh);
        continue;
    }

    $block = 1024*4;
    while (true) {
        $data = fread($fh, $block);
        if ($data === false || empty($data)) {
            printf("read failed %d %d\n", $seek, $block);
            @fclose($fh);
            continue 2;
        }
        $res = fwrite($fh_out, $data);
        if ($res === false) {
            printf("failed to write?\n");
            continue 2;
        }
        unset($data);
        $data = null;

        $seek += $block;
    }
}
fclose($fh_out);