A Lumia 1520. I think the message said something along the lines of "next time you remove your SSD card to first ..." That's not exactly it but it was telling me how to properly remove the card. Even though I had not.
It has me a bit worried about my SSD card. At least everything there is on OneDrive.
I started getting the same message about once per day on my 521 with a SanDisk Ultra 32GB class 10 that's about 2 years old. I tried reseating the card a few times, but I'm still getting the message. I'm wondering if the card has just had too many 'read-write' cycles and should be replaced.