scandisk.pl

This script will read a file (or raw character device, such as /dev/rdisk0 on a mac), looking for read errors. It tries to work in large chunks to reduce the system call overhead, but will bring itself down to 512 byte blocks to accurately report how far into the (file or disk) the read errors are happening. It also reports on slow reads (>3 seconds).

http://gigo.com/ftp/pub/src/scandisk.pl