Short: Free & marvellous Recuva - File Recovery software from Piriform worked for me when a substantial number of other free and for-$ programs failed to.

The program is currently Windows-only but even a hardcore Mac man can probably stoop to using a Windows machine in such dire circumstances :-).

Some notes at end or see their website as above.

Fully free (as in beer) - free to download and no $payable to access results.

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Longer:

I recently rescued a large number of holiday photos from a friends SD card after an unknown cause crash.

I tried a range of free and paid software (many allow a free analysis and they then charge you to unlock the recovery if successful). None worked - some would locate a small handful of files and then declare the card empty or corrupt or whatever.

When all seemed lost I tried the unencouragingly named "Recuva" software from Piriform an achieved complete success. Initially it reported similarly to others but it had an "I'll try harder option" (they call it "Deep Scan") which produced several thousands of files ranging from full recent images to thumbnails going back many many prior photograph-download cycles by the user.

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They say:

Recuva brings lost files on your computer, USB drive, camera or iPod. More...

Recovery from damaged or formatted disks:
Even if you've formatted a drive so that it looks blank, Recuva can still find your files on it.

Recover deleted emails:
Emptied your email trash and need it back? Recuva's got you covered with full support for Microsoft Outlook Express, Mozilla Thunderbird, or Windows Live Mail.

Recover deleted iPod music:
Deleted music from your iPod or MP3 player? No problem, Recuva will get this back for you along with any additional track data.

Restore unsaved Word documents:
Did Microsoft Word crash or did you forget to save that important Word document. No problem with Recuva! As it can intelligently rebuild Word documents from their temporary files.

Quick-Start Wizard: If you need your files back right away without fiddling with options, Recuva's Quick-Start Wizard is the answer.

Deep Scan: Recuva can find most files within a minute. Or, set the Deep Scan to look for more deeply-buried results.

Securely delete files you want to erase forever Want to protect your deleted files? Recuva also allows you to permanently erase any traces of deleted files.

Portable version Take Recuva with you wherever you go with the portable version.

Full Windows OS support and many languages Recuva has support for every modern version of Windows and 37+ languages.