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If you’re using macOS 10.13 (High Sierra) on a machine whose internal disk is an SSD, or you ran the High Sierra developer preview at some point prior to its release (on a machine with any type of internal disk), High Sierra will have converted your internal disk to use the new Apple File System (APFS).
iDefrag does not support APFS, and will not display disks formatted with it as options in the volume list. At time of writing, Apple has still not published the technical specifications for APFS, and even when it does, implementing support for a new filesystem in iDefrag is many months’ work.
If you are seeing the disk in the volume list but the block display doesn’t appear when you click the lock, the problem is instead System Integrity Protection blocking iDefrag from accessing the boot volume. In that case we recommend that you make a boot disk. We do not recommend disabling System Integrity Protection, as it provides valuable additional security for your machine.
Posted by alastair at 2017-Nov-29 14:11:28 UTC. Last updated 2017-Nov-29 14:11:42 UTC.