So I struggled with this for a long time, looking for different topics with different solutions, but I have not yet managed to merge my scores into one. I had previously installed Windows 10 on this one, but after partitioning for a mutually accessible drive (for Dropbox), Windows got stuck and I had to reformat it, which gave a sort of partition "locked".
If I click on "Partition" in Disk Utility, there are two main partitions, where none of them have the option "-" available (dimmed). I've tried to partition DeleteThis into ExFat, Fat and Journaled OS X, without any difference.
disk utility list reveals this:
/ dev / disk0 (internal, physical): #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme * 1.0 To disk0 1: EFI EFI 209.7 MB disk0s1 2: Apple_CoreStorage Macintosh HD 885.3 GB disk0s2 3: Apple_Boot Recovery HD 650.0 MB disk0s3 / dev / disk1 (internal, physical): #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme * 121.3 GB disk1 1: EFI EFI 209.7 MB disk1s1 2: Apple_CoreStorage Macintosh HD 121.0 GB disk1s2 3: Apple_Boot Boot OS X 134.2 MB disk1s3 / dev / disk2 (internal, virtual): #: TYPE NAME SIZE IDENTIFIER 0: Apple_HFS Macintosh HD +997.8 GB disk2 Logical volume on disk1s2, disk0s2 F4F65E7F-70D0-4958-958E-6272758C056B Fusion Drive unencrypted
I've also tried restarting in recovery mode and using the disk utility, with no results different from the normal one.
EDIT: Of course, I also tried using Boot Camp Assistant, but it just says, "The boot disk can not be partitioned or restored as a single partition."