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.
Best Answer
I can't find a specific Apple KB reference, but the consensus seems to be that you need Firewire, Thunderbolt or USB-C; USB 2 doesn't appear to be supported.
From the MacSales blog - Mac OS 101: How to Boot and Use a Mac in Target Disk Mode