We have a Mac mini connected to an external storage box via FireWire. In a reasonably sensible manner, someone pointed Time Machine at one of the volumes on the DAS, thinking they should back the machine up to this location. They also thought also it would be best to encrypt the backup; very laudable.
However, this triggered a conversion process into an Apple Core Storage volume, which did not complete successfully. Luckily, OS X stopped trying and remounted the volume as HFS+. All appeared to be well, but after a reboot things were much less cheerful.
On restarting the machine, the volume was missing. Checking with
diskutil list returned this layout:
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *17.6 TB disk1
1: EFI EFI 209.7 MB disk1s1
2: Apple_HFS Backup 2.0 TB disk1s2
3: Apple_CoreStorage 15.6 TB disk1s3
4: Apple_Boot Boot OS X 134.2 MB disk1s4
diskutil cs list returned this: