I'm trying to create a 2nd partition on Late 2012 iMac with 3TB Fusion Drive. In disk utility, I select the disk drive, select Partition, click the +, and drag the new 2nd partition size down to 252.5 GB. Then hit Apply. Note that the disk Size field is greyed out throughout this process so you can see the size displayed, but you can't type in the Size field to directly specify the size. You can only drag the disk size up and down. But after the partition is created, it is 834.8 GB in size, not 252.5 GB. Is there a minimum partition size, bug, other?