Checking if any data exists on a presumably empty storage device

There is a device, /dev/zero on a linux system that always gives zero when read.

So, how about comparing your hard drive with this device:

cmp /dev/sdX /dev/zero

If all is well with zero-ing out your hard drive it will terminate with:

cmp: EOF on /dev/sdb

telling you that the two files are the same until it got to the end of the hard drive. If there is a non zero bit on the hard drive cmp will tell you where it is in the file.

If you hav the pv package installed then

pv /dev/sdX | cmp /dev/zero

will do the same thing with a progress bar to keep you amused while it ckecks your drive (the EOF will now be on - rather than sdX though).

From https://superuser.com/a/559855/236344:

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