Restoring a HDD backup image from the Ubuntu Live CD

Question

I want to restore a HDD image I have to my laptop's HDD while booted off the Ubuntu Live CD.

The laptop's HDD is unformatted and has no partitions.

I expected this to work:

$ sudo dd if=/path/to/backup.img of=/dev/sda

But I'm told

dd: opening `/dev/sda': Permission denied.

What gives?

Answer 1

Hmm. Doing a su first worked. Why's this?

$ sudo su
# dd if=/path/to/backup.img of=/dev/sda