This Question - Getting loads of Open error 5's when using Terminal to repair permissions - raises the question,

Does mounting a drive in Target Disk Mode bypass SMART, preventing any lockout if the disk is failing & you need to recover data?

In the past, I have always put suspected failing drives [often Windows NTFS drives] in a USB enclosure & attached to my Mac to scavenge/repair, as that way I am ignoring both SMART & any OS intervention from Windows etc.

For a Mac, would Target Disk Mode be just the same as an external USB for similar tasks, or could SMART potentially restrict operations?

Edit: Google would seem to think that SMART reporting cannot be carried over USB or Firewire, but that doesn't exclude the possibility of Thunderbolt. I cannot find any canonical answer even to the first two, though.


S.M.A.R.T is a monitoring system for HDD and SSD implemented in the firmware of a drive using two different standards: ATA and SCSI.

SMART data is usually written to a special service part of the platter/flash storage unit.

All modern interface standards and the underlying protocols (SCSI, FireWire, USB – with USB-(S)ATA-Bridges, eSATA, Thunderbolt, SAS) are basically capable of transmitting SMART-data.

SMART data is not actively transmitted to hosts, but has to be queried by the host-OS, applications or drivers.

Depending on the OS, the SMART application, the drivers and the build-in hardware the success of those queries may vary broadly. BTW to retrieve SMART data from external USB/FireWire-drives attached to Macs use this kext driver (compatibility list).

As a result Target Disk Mode doesn't 'bypass' SMART at all, because SMART is implemented in the hard disk. It's hard to say in general if operating systems (or application and drivers) are locking out failing disks based on SMART data.

I doubt that at all because

  • SMART isn't very reliable
  • the implemented attributes varies between manufacturers and hard drive models
  • the meanings and interpretations of the attributes and their values varies between manufacturers

At least i don't know of any such OS or application.

It's rather the OS in conjunction with the firmware and the (failing) hard disk itself which 'lockout' a device.

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