I have a late 2013 Mac Book pro with a 500GB HD. I needed to downgrade my OS from High Sierra to El Capitan. After doing this, I began installing all the software I use back on it an noticed my hard drive was now 250GB. diskutil cs list returned this drive as being 250GB and another partition containing Apple_Boot Recovery HD (about 2GB) with a capacity of the other (250GB) that where missing. System report also showed the disk was 500GB and had two partitions of 250 each. Nontheless, this drive was unaccesible from disk utility (even with Debugging) and there was no way of repartitioning the drive, so I decided to format the computer again from a USB drive and try the process from scrach. This time, however, that other half disappeared completely. I cant see it anywhere, not even the terminal in recovery mode, system report or disk utility with debugging enabled. In summary, I've been left with a drive with half its capacity and the other half has gone completely missing. any suggestions/ideas? thanks!