How to Partition A Fusion Drive

See Controlled Fusion: Partitioning for Targeted Performance.

Shown below are two partitions of the Fusion drive in Disk Utility.

To partition a Fusion drive:

1. Backup your stuff (two backups strongly recommended).
2. Boot off any working drive (e.g. one of your backup clones).
3. Erase the Fusion drive (because your stuff might not fit on a downsize partition).
4. In the Partition tab, click the + button.
5. Drag the slider between the partitions to size as desired.
6. Set a name for the 2nd partition.
7. Apply.
8. Clone your system back to the new Boot volume (if too large, move some items first to the new 2nd volume).
9. Copy your other data (data not going onto the Boot volume) onto the 2nd volume.
10. Set the startup volume to the new Boot volume, reboot.

Note that Disk Utility has a bug in displaying the actual volume size in the Partition tab, the actual size produced as shown here is 186GB, not 195GB).

As shown, the volume “helios” is a PCIe SSD, used as the boot drive used while doing the partitioning; one must boot off a different volume in order to rejigger the Fusion drive— cloning is fast and easy, see also How to upgrade your system/boot drive.

Fusion drive partitioned for more SSD for Boot volume , remainder as hard drive (hdd)

Fusion drive partitioned for more SSD for Boot volume , remainder as hard drive (hdd)

Actual SSD and HDD drive sizes in the late 2012 iMac with Fusion 1tb are as follows:

APPLE SSD SM128E: 121.3 GB = 121332826112 bytes = 118489088 1024K blocks
APPLE HDD ST1000DM003: 1000.2 GB = 1000204886016 bytes = 976762584 1024K blocks