I think this problem occurs at windows 8. I am not sure but testdisk may have compatibility problems with windows 8.
i had recovered lots of partitions with testdisk before. all of them had done at windows xp.
but now the disk structure of my 1.5TB hdd is corrupted and it is couldn't be able to recovered while working at windows 8. i am trying for a week. i have a lot of important data in it.

i have the same error with you friend. while i had been trying at windows 8, i had never take any error like this.
i will install windows xp and try on it. i think you should also try this.
for sharing help,please add me on skype
mustafayanci@barismancomix.com