I've been having a minor problem with MC11 wiping out my entire library.
A while back, I moved my entire music collection onto an external 200GB hard drive. I also placed the library files there as well.
I then got into the habit of turning on the external hard drive only when I planned to play music.
That was a mistake - because occasionally, I'd launch MC11 without the external hard drive turned on. MC11, not being able to "see" the library, would ask if I wanted to use some other old library that it had found. I click Cancel, and get the import files dialog box. I click cancel and get an empty library.
At that point, my library was lost. Turning on the external drive, relaunching MC11, and even rebooting wouldn't restore the library. The database files seemed to still be there on the external drive intact, but I couldn't figure out how to get MC11 to see them again. If there was a way, please let me know.
I've done this several times, and each time I've reimported my entire library, and reconstructed my view schemes and smartlists. I've finally gotten into the habit of backing up my library regularly. And I just moved the library files to my boot drive to avoid this problem entirely.