[Shield Tablet Q&A, Help & Troubleshooting] badytosk:
I believe /data is ext4

root@shieldtablet:/ # mount | grep /data
/dev/block/platform/sdhci-tegra.3/by-name/UDA /data ext4 rw,seclabel,nosuid,nodev,noatime,noauto_da_alloc,data=writeback 0 0

Full mount output (if it helps):

127|root@shieldtablet:/ # mount
rootfs / rootfs ro,relatime 0 0
tmpfs /dev tmpfs rw,seclabel,nosuid,relatime,mode=755 0 0
tmpfs /dev tmpfs rw,seclabel,nosuid,relatime,mode=755 0 0
devpts /dev/pts devpts rw,seclabel,relatime,mode=600 0 0
none /dev/memcg cgroup rw,relatime,memory 0 0
none /dev/cpuctl cgroup rw,relatime,cpu 0 0
proc /proc proc rw,relatime 0 0
sysfs /sys sysfs rw,seclabel,relatime 0 0
selinuxfs /sys/fs/selinux selinuxfs rw,relatime 0 0
debugfs /sys/kernel/debug debugfs rw,relatime,mode=755 0 0
none /sys/fs/cgroup tmpfs rw,seclabel,relatime,mode=750,gid=1000 0 0
none /sys/fs/cgroup/memory cgroup rw,relatime,memory 0 0
pstore /sys/fs/pstore pstore rw,relatime 0 0
none /acct cgroup rw,relatime,cpuacct 0 0
tmpfs /mnt/asec tmpfs rw,seclabel,relatime,mode=755,gid=1000 0 0
tmpfs /mnt/obb tmpfs rw,seclabel,relatime,mode=755,gid=1000 0 0
tmpfs /mnt/tmp tmpfs rw,seclabel,relatime,mode=775,gid=1003 0 0
/dev/block/platform/sdhci-tegra.3/by-name/APP /system ext4 ro,seclabel,relatime,data=ordered 0 0
/dev/block/platform/sdhci-tegra.3/by-name/CAC /cache ext4 rw,seclabel,nosuid,nodev,noatime,nodelalloc,data=writeback 0 0
/dev/block/platform/sdhci-tegra.3/by-name/UDA /data ext4 rw,seclabel,nosuid,nodev,noatime,noauto_da_alloc,data=writeback 0 0
/dev/block/platform/sdhci-tegra.3/by-name/RP3 /mnt/usercalib ext4 rw,seclabel,noatime,data=writeback 0 0
/dev/block/platform/sdhci-tegra.3/by-name/FCT /mnt/factory ext4 ro,context=u:object_r:factory_file:s0,relatime,data=ordered 0 0
/dev/fuse /mnt/shell/emulated fuse rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other 0 0
tmpfs /storage/emulated tmpfs rw,seclabel,nosuid,nodev,relatime,mode=050,gid=1028 0 0
/dev/block/vold/179:33 /mnt/media_rw/sdcard1 vfat rw,dirsync,nosuid,nodev,noexec,relatime,uid=1023,gid=1023,fmask=0007,dmask=0007,allow_utime=0020,codepage=437,iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro 0 0
/dev/block/vold/179:33 /mnt/secure/asec vfat rw,dirsync,nosuid,nodev,noexec,relatime,uid=1023,gid=1023,fmask=0007,dmask=0007,allow_utime=0020,codepage=437,iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro 0 0
/dev/fuse /storage/sdcard1 fuse rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other 0 0
/dev/fuse /storage/emulated/0 fuse rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other 0 0
/dev/fuse /storage/emulated/0/Android/obb fuse rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other 0 0
/dev/fuse /storage/emulated/legacy fuse rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other 0 0
/dev/fuse /storage/emulated/legacy/Android/obb fuse rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other 0 0

Edit2: I've again formatted absolutely everything that I could via the latest multirom... I've made sure that everything is ext4. I then flashed multirom + touch, then newest blisspop. Mutlriom works good, Internal boots. I've reboot to recovery again, added the latest (yesterdays) ubuntu rom. I have still the same problem as described above... Does anybody have any idea what can I do to make this work?

Edit3: Removed the rom from multirom list, installed to external card - now when I reboot the multirom doesn't find it.... Only shows Internal... I'm desperate :(