SCSI passthrough on OS X
Support for external USB or FW drives is currently lacking on Mac OS X because that OS does not provide SCSI passthrough natively. There is however a kernel extension that promises to add exactly this: https://github.com/kasbert/OS-X-SAT-SMART-Driver .
It would be great if the appropriate code could be added to os_darwin.* to support external drives under OS X also. I've taken a quick look at the code, but it doesn't look to be an easy fix without a serious bit of introduction...