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On High Sierra (macOS 10.13), if you click on the boot disk in iDefrag, then click on the lock to unlock it, you will see the message
There was a problem opening this volume
This happens because Apple has tightened security on High Sierra such that only Apple software is now able to directly access the boot disk; it is possible to disable System Integrity Protection (the mechanism being used to prevent access), but we don’t recommend that — it’s a valuable protection and makes it much less likely that malicious software will e.g. empty your bank account for you.
Fortunately, because this extra security only applies to the disk you actually booted from, you can make a boot disk and start from that instead as a workaround.
Posted by alastair at 2017-Dec-21 13:12:08 UTC. Last updated 2017-Dec-21 13:12:45 UTC.