Fixing the "SD Card Unexpectedly Removed" error on Android
Recently, on a Lenovo Yoga Tablet 2 Pro (model 1380F) running Lollipop 5.0.1 with a microSD card inserted, I began seeing the "SD Card Unexpectedly Removed" error every time the screen powered off. The card hadn't been moved from the slot in a while and other than routine app updates there were no software changes that could have affected the card. Even more strangely, the SD card was no longer detected at all or mountable in the settings -> Storage menu, even after manually removing and re-inserting it. Only a reboot would get the SD card working again.
A Google search of this issue reveals that there may be several different causes. At least in my case, it turned out to be a filesystem issue. After connecting the microSD card to my Windows desktop via a USB reader and running
chkdsk, things returned to normal on the tablet! So if you're experiencing this error, try putting the card in your computer and running a filesystem scan - it just might fix it.