You've build a large world and want to back up it, so you save the world on you memory.
Minecraft gives an alarm successfully saved or similar. If you close minecraft now, your save will be 0byte and useless. The only way to be Shure is to watch the new file in Windows Explorer until it gets bytes, so it occupies a memory size.
Thank you for your report!
We're tracking this issue in MCPE-41898, so this ticket is being resolved and linked as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki
-- I am a bot. This action was performed automatically! The ticket was resolved by one of our moderators, and I left this message to give more information to you.