Mount 'data' partition in Debian

I’ve flashed the Debian build and currently playing with it.
I suppose the partition ‘userdata’ (/dev/sdd13) can be mounted to provide more space.

I’ve also supposed it contains the ext4 partition (from Android) but it is not (nevertheless it contains some data).

At the same time /data/sdd12 (partition ‘reserved’, label data) seems contains ext4 filesystem but mounting it is failed with kernel message:
EXT4-fs (sdd12): bad geometry: block count 6291456 exceeds size of device (256 blocks)

BTW, testdisk also sees something in /dev/sdd12 (label ‘data’):
Disk /dev/sdd - 31 GB / 29 GiB - CHS 3811 64 32
Partition Start End Size in sectors
P MS Data 70912 87295 16384 [BOOT]
P MS Data 94976 1295103 1200128 [rootfs]
>P MS Data 1495808 7787263 6291456 [data]

So, what’s wrong here?
Should I correct /dev/sdd12 somehow? or should I instead make new empty filesystem on /dev/sdd13 ?

Based on my notes, I think you should execute below command with fastboot command:

sudo fastboot format userdata

Please NOTE, this command will format the whole userdata partition and all the old content in the data partition will be lost, so please be careful for this.

Maybe you also can try below command in Debian system:

# mkfs.ext4 /dev/sdd13

I’ve tried this couple days ago (the command was ‘mke2fs -t ext4 /dev/sdd13’), this works pretty good.

(Although the presence of some huge ext4 in ‘reserved’ is not clear. Seems like ‘reserved’ was actually a beginning of ‘userdata’ but partition offsets were changed later. Maybe UEFI bootloader did this? Does not matter thought…)