I see this issue occurred 2015, but just recently noticed this happen to my spare handset S4 used for special access to apps and data.
Here's what I think happened to cause SMemo to vanish.
I had recently added One Note by Microsoft, and this app has very nasty actions built in to not allow any data saved to it to be allowed or shared to other note applications.
I Truly hate OneNote. It acts all weird, where it flexes while in view like an image.
Point in case, LG V20 handset, with any attempt to paste data from One Note, it instantly crashes built in Quick Memo App.
I also tried to paste data to newsgroup I use, and the same thing happens.
So yesterday, I looked for a note app to try, that is not sensitive to One Notes action.
'NotePad Free' at Google Play works perfect, and you'll see my review there as well how astounding simple this guy developed it.
Where 'One Note' captures entire web page as you saw it, and difficult to edit, NotePad Free just saves the text.
I will also try hints others here suggested to see if update wiped out the SMemo App.
SMemo App is still present, but the hundreds of notes have vanished.
I was able using computer app on phone to locate a saved note, and it was able to import it back in.
But by default, the application should alert where to save data, to SD CARD.
I SUGGEST 'EVERNOTE' APP.
I JUST UPGRADED TO FULL VERSION ACROSS ALL DEVICES.
— modified on Mar 28, 2018, 10:28:18 PM