ok i managed to save the error log. Interestingly enough, if I connect the Macbook Pro to my iMac it doesn't work, but if I connect it to my Macbook Air (both times with Target Disk Mode via Thunderbolt cable) it works fine.
anyways here is the error log http://goo.gl/SMpx06
I tried connecting an SD card to save the error log, so there might be something about that in there.
EDIT: even on the Macbook Air the disks disconnected after a couple of minutes, with the usual red "The disk was not ejected properly" error