My phone suddenly shows an SD-card notification My Sony Xperia J phone was in my pocket and I wanted to read an ebook, so started my reader application. It showed a black screen and when I checked the shelf, all my books were gone.

I noticed a new notification icon that looked like a folded SD card. I checked its message that said: SD card is empty or its file system is not supported. 

What? Half an hour ago I was using it.
I tapped the message and a question appeared: Do you want to format your SD-card? It'll erase all data from it. 

What?? All my ebooks and media are gone just like that? 

I restarted the phone, still no SD-card. When I got home, I removed the card and reinserted and fortunately now it works.

How does that so-called OS dare to lose my data? And the "recovery" solution is to format the card?

What could cause this?
One of my family members had the same problem, SD card misbehaving, then working for a few days, on and off. After a week or so the phone did not read the card at all and neither did the PC. Fortunately I backed up the content of the card after the first problem. The card worked for about a month after formatting, but died eventually.

I do not know what causes this.
I do not know how to stop it.
I know what you should do: a backup.