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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?

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Are you sure there are no partitions? Check back with sudo fdisk -l and mount, maybe something got mounted 'by accident'. – Bobby Feb 3 '10 at 13:05
@Bobby Yeah - I got it working, see my answer, but don't understand why. – xyz Feb 3 '10 at 13:13

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

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