Delete CoreStorage Volume even as OS X won't acknowledge there is one?
I ran the command cat /dev/random &gt; /dev/disk2 as user "su" and all partitions are gone now. At first, the available disk space was still only 801GB. After a while it showed up properly in Disk Utility as 3.0TB though and I was able to reformat it. Excellent!

Thanks to Topher Kessler for posting the proper commands in How to fix deep formatting problems with OS X drives!

CoreStorage is great technology. Apple info still leaves a lot to be desired.