So i've battled with this for quite some time, look up various topics with various solutions, however i have yet to succeed with merging my partitions into one. I had previously installed windows 10 on it, but after i partitioned for a mutual accessible drive (for dropbox), windows stalled and i had to reformat it, resulting in a sort of "locked" partition.
If i click on "Partition" in Disk Utility, there are two major partitions, where neither of them have the "-" option available(greyed out). I have tried to partition DeleteThis into both ExFat, Fat and Journaled OS X, with no difference.
I've also tried to reboot into recovery mode and use the Disk Utility there, with no different result than from normal.
EDIT: I have of course also tried using Boot Camp Assistant, but it just says "The start disk cannot be partitioned or restored as a single partition".