Stskeeps helped me out.

This was when I was on the 10.0.1.x version of the os.

The
mount: mounting /dev/mmcblk0p28 on /mnt failed: Invalid argument

Error message is most likely due to corrupted filesystem on /dev/mmcblk0p28

Maybe you should contact Jolla Care ?

If you are able to get a prompt when connected to the device, its fixable.