My hard drive’s crashed!
Macs running OS X Mavericks that have Western Digital external hard drives connected and are running WD utilities are known to have experienced problems where the drive is corrupted and data apparently wiped. Some Macs without the WD utilities installed have also had problems, and in one case we know of, a non-WD hard drive on a Mac running Mavericks and WD utilities was re-formatted and named ‘My Book’.
Whichever OS you’re using and whatever hard drives you have connected, back them up regularly. An issue like that one comes out of the blue and can cause catastrophic data loss.
If you only use external disks occasionally, disconnect and power them off when you don’t need them.
Already lost data to this or a similar problem? Don’t use the re-formatted drive. Use a data recovery tool like those described above to scan, catalogue, and recover lost files. When it comes to recover files, you’ll have to open each one to identify its contents and then rename it accordingly. Quick Look is useful for quickly previewing files, which can then be re-named in the Finder.