How to Store BTsync Data Files in SD Card?
Answer from the BTsync support

Did you disable 'Simple mode&quot; in Sync settings -&gt; advanced? If yes, then after scanning a QR code from another peer, to which you want to connect, Sync will ask you to pick folder location, and it's here where you'll be able to pick SD card. Please see detailed guide here.
Else, you can start sharing from the mobile - tap on &quot;=&quot; icon -&gt; add folder and pick a folder from the SD card, share it with other peers. Detailed guide is here.

where disabling the Simple mode is the trigger to solve my case.
Then Settings &gt; Default folder location and you are able to change to SD card.