Hello,
I am new with /e/OS so maybe my question was already answered.
Not even sure, I am posting into the right category 
How could I encrypt SD card, so only my phone could read the content ?
I have murena one phone, but maybe it is general question.
I was expecting this feature will be part of the /e/OS. Maybe it is, but I could not find it.
Thanks for asnwers
Regain your privacy! Adopt /e/ the unGoogled mobile OS and online services
You would usually just format the SD card as extra phone storage (a.k.a. extended internal storage) after having inserted it into and booted the phone. This will make the card contents accessible only from within your phone.
However, in the past this has caused functionality issues with some phones like the Fairphone 3. I was meaning to post a screenshot here, but currently the forum won’t allow me – last time I formatted an SD card the screen would show a warning against extra phone storage:
Warning: This option may not work properly or lead to data loss and is therefore not recommended!
I don’t know if these problems affect the Murena One as well. Maybe others can contribute something to provide experience if it works out now. You might just want to insert the card and see if the above warning still shows when you are prompted to make your choice (the other option is phone storage, but this is meant to allow the card to be accessed in other devices as well).
The internet suggests it can hit you with any device, if you’re unlucky. It seems to be an issue of recent Android in general.
The Fairphone 3/3+ is affected by something which somehow makes the matter much worse.
AOSP docs use the terms traditional and adoptable storage… see https://source.android.com/docs/core/storage/traditional
Only adoptable can be encrypted as of writing, and as my prior posters noted, if the sdcard is somehow corrupted when adopted, you’ll have to go full reset.
So the current state is a bit sad, you’ll need to use 3rd party apps to create encrypted storage on the “traditional” sdcard. “EDS Lite” App comes to mind. Then there are modules for LUKS that you can employ if you have root - but not straightforward: