Why did an "Untitled" partition appear that uses up half the disc space and how do I get rid of it?

You only have bookkeeping files on that partition. Unless there are hidden files, you can delete it and expand the / partition to get all the space to use.

sudo ls -laOR /Volumes/Untitled

As to why, that might be hard to guess unless someone was contemplating Bootcamp or making a RAID and needed to reduce the size to match the other disk.

To delete it, Disk Utility is the easiest method to resize the main partition once you've deleted the Untitled one. Just read the dialog carefully on which partition's data will be erased and which will remain before pressing OK in the dialogs.

You can learn more here:

• How to resize Macintosh HD?

The command you might need to do that from the command line without prompts asking if it's OK (so be sure to back everything up on the mac first) would be:

diskutil mergePartitions JHFS+ "Macintosh HD2" disk222s2 disk222s3

I made the command use disk222s2 to avoid it working on all reasonable setups. If your second drive was disk2 that held the splits, you would substitute disk2s2 disk2s3 after checking the s2 and s3 parts matched the volumes to merge.