SCSI devices inaccessible e.g. due to a SCSI reset
Posted by Sven Koester, Last modified by Andre Kuehnemund on 10 March 2011 10:03
The operating system may under some circumstances loose the connection to the tape drive or the media changer on the SCSI bus and cannot reconnect them on its own. The following events may cause that behaviour:
  1. The door of the media changer has been opened. Some changers and/or tape devices disconnect from that SCSI bus in that case. A disconnected device is not detected after reconnect by Mac OS X. This is the case even if the ioreg utility shows the device . The ioreg utility gets its data from a cache that is set up during boot time, and is not aware of changes to the SCSI bus.
  2. During operation, a SCSI reset appeared. This implies that all devices are disconnected by Mac OS X. The reason for a SCSI reset may be a defect or dirty tape drive.
  3. A device was powered down, e.g. by a power loss or similar.
  4. The SCSI chain is too long, or the SCSI cable, terminator or adapter is faulty.
Often, SCSI problems are visible in the system log.

To bring back the device to be usable, OS X must be restarted.  Please follow these steps to do so:
  • Bring down Mac OS X
  • Switch off the media changer or tape device
  • Switch on the media changer or tape device and await it's initial startup
  • Start Mac OS X
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