recovery recommendations

[m.p.]

Recently a 640GB external enclosure was PEBKAC'd with the following
command:

dd if=/some-185mb-linux-install.iso of=/dev/sdx

[not of=/dev/sdx1]

Since the PEBKAC, the drive has not been written to beyond being
unplugged. I have made an image of the drive and have attempted to run
against it: testdisk, fsck.ext3 with alternate superblocks, and even
"fsck.ext3 -Sy", to no avail.

So. I am *convinced* that my 550gb of data is recoverable. It seems that
[obviously] the first 185mb is gone - whatever files those were.

I have googled, but to be honest, I am not entirely sure I'm searching
the right strings. Most software seems geared towards partition recovery
where a partition has become damaged [which is technically what's
happened], but not exactly; or partition undeleting [which again isn't
technically what happened].

Please, any recommendations that don't involve a time machine are much
appreciated.

If this isn't the place to be asking, please point me in the right
direction.

Thank you.

-mp- - kicking self -