How to recover a System partition that won't mount

A relative of mine brought me his *Galaxy S4 Mini GT-I9190*, which is stuck at boot. When I boot into recovery it gives the error Failed to mount /system (invalid argument). How can I dig through the file-system of an android phone which doesn't boot, except in Recovery? The goal is not to flash it but to recover the data if possible. I don't mind rooting or flashing, but I don't want to overwrite the system partition that may still have the valuable info on it. I've managed to install clockworkmod according to these instructions . **Is there a way to mount it, or expose the raw device to USB, or just back it up to SD, _without_ overwriting it?** **EDIT:** Now I have Clockworkmod installed. Trying to use it to make a backup of /system. It fails to mount /system with the message: mounting /dev/block/platform/msm_sdcc.1/b/system on /system failed: Invalid Argument W:failed to mount /dev/block/platform/msm_sdcc.1/by-name/system (File exists) **EDIT 2:** with ADB shell I looked into /system, found an empty folder called bin and removed it. Now mounting /system fails with a longer error: Error mounting /system! W:failed to mount /dev/block/platform/msm_sdcc.1/by-name/system (File exists) I:Can't partition non mmcblk device: /devices/platform/msm_sdcc.3/mmc_host