problems with SDCard after partitioning

Question

I'm using a rooted XPeria Z1c. Recently I got myself a bigger SDCard (128 GB) and I decided to use 2 partitions. I started 'adb shell' then partitioned the card using

sm partition disk:<disk_id> mixed 50

After disconnecting the phone I formatted the 2nd partion as exFAT. Windows' disk manager is showing me 3 partitions on the SD: one exFAT plus 2 more that obviously aren't recognized. One of those 2 is quite small (16MB in size), the other one has roughly the size I'm expecting it to have.
EDIT: formatting was done by the phone itself after I first inserted the freshly partitioned card, and I expected the format to be ext4.

My phone has no problems using both of the main partitions, so that's good.

On my PC I'm running a Linux device driver by Paragon enabling me to access extFS formatted sd cards an pendrives from Windows Explorer. But if I directly connect the card to my windows PC (using an SD card reader) only the exFAT partition is accessible. Apparently the other partition is not truly formatted as ext4.
I tried various adb / terminal commands to access that special partition and find out what it is but I can't really access it (fdisk, mount, cat, df, ...).

Can anyone give me an idea how to handle this? What file system could be used for my internal SD card if it's not ext4?

EDIT2: (after comment from @Izzy): I just created a new (smaller) SD card with just one partition and formatted as ext4. After inserting it into my phone I had to prepare and format it as internal storage. After that I pulled it off my phone again and examined it using my PC. Result: the ext4 partition is readable in Windows (through the Paragon driver) and it also has an extra 16MB partition. No idea whatsoever what this is used for...

EDIT3: doing some more experiments with my smaller sd card I think that @Izzy pointed into the right direction; might as well close this question

— ElEm | Sources

Answer

As it turned out, your partitioning was due to the fact you wanted to use adoptable-storage (and not, as one might assume, to use link2sd). As our adoptable-storage tag-wiki points out, adopted storage will be encrypted â€“ not just the files on it, but the entire partition. That explains two things:

attaching the card to your PC using a card reader, the PC cannot read the partition without having the encryption details. As you didn't provide them to the PC, it could not "open" it

attaching the device via MTP however shows the partition fine on your PC, as the Android device has decrypted it

Remains the mystic 3rd partition. The small size suggests it's some "control partition". A good guess as it turns out: if you list the label, you will find it's called android_meta (while your adopted partition will be labeled android_expand) â€“ which you can find out e.g. by a Google search for "sm partition disk" 16m. Quoting from this Oopsmonk article on how adoptable storage creation works:

It will create two partitions (android_meta and android_expand) for adoptable storage, the android_meta is a reserved space for feature use and android_expand is the external storage which is encrypted via dm-crypt.

With that last detail, your puzzle should be complete

— Izzy | Sources

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