General SCSI and USB Troubleshooting



With a lock up on the backup computer you want to try moving the mouse to see if the ADB or USB bus has locked up as well.

If you are using a USB device, and the mouse is frozen, this is often the indication of a USB hang. Try the USB device connected directly to the computer rather than via a Hub. Try unplugging the mouse and plugging it back in again. Try unplugging the USB tape drive and plugging it back in again. Update to the latest USB drivers and device firmware.

If the mouse moves but the Mac is hung up it’s most likely that your SCSI bus is hung and you have a SCSI problem. If the mouse doesn’t move with a SCSI device that means that the Mac’s processor is hung and you should look at System Software and extensions. More often than not you will find that the mouse moves and you can force quit the program (at which point the Mac may or may not completely crash).

Macintosh and Windows

A SCSI or USB hang or error can be caused by one or more of the following:

  1. A dirty tape drive or bad tape. Clean the drive using a cleaning cartridge. Your drive manufacturer recommends that you clean the unit once for every 8-10 hours of run time. Once a week is more than enough for most people. Try another tape. If other tapes work then you just have a tape with a spot that’s bad enough to crash your backup.

  2. Another device on your USB or SCSI bus is interfering with the tape drive’s communication. Make sure your SCSI ID numbers are set correctly. Turn off your computer and the devices. Disconnect all SCSI devices except for the tape drive. Try the USB backup device connected directly to the computer

  3. You have a bad cable. Replace the SCSI or USB cable that connects the tape drive to the computer after removing other devices and cables from the SCSI or USB chain.

  4. You are missing a terminator (SCSI) or have a bad terminator. The last device and ONLY the last device in your SCSI chain needs to be terminated. Try replacing the terminator if you already have one on the chain.

  5. The computer may be having a problem. Install Retrospect on another computer and try the tape drive there as the lone device on the USB or SCSI chain.

  6. The drive may be defective. If you have implemented all of the preceding steps and get failures on multiple tapes after changing cables terminators and computers then the drive (being the only factor that has not changed) is the culprit—​send it back to your vendor for repairs.

The steps above are essentially the outline of our device troubleshooting. Hands on testing of device issues is really still the best method and even getting device logging information is usually only to confirm empirical testing. Note that concluding something is a bad device is the LAST thing we assume after all other components and variables have been ruled out. "SCSI voodoo" as they call the nebulous symptoms that can plague a SCSI bus can often lead one to false assumptions of the cause of problems.

It’s important that once a variable is tested that it be tested more than once for consistency’s sake to rule out dumb luck. For example SCSI voodoo accounts for why a tape drive may work fine for many months without proper termination but then suddenly fail in some way later. Although customers will often cite that nothing has changed with their SCSI bus configuration in months and that it was all working before this is really indicative of the inconsistency of SCSI voodoo.

The quickest and most conclusive test for most devices is to test it on more than one computer as the only device on the bus and with a different SCSI cable. If the problems can be reproduced on multiple computers it’s more than likely a hardware problem with the device itself. Of course there a myriad of other specific issues having to do with a device’s own hardware settings like with internal jumper cables dip switches or internal termination that has to be sorted out with the device’s manual and/or vendor or manufacturer of the drive, but the kernel of SCSI troubleshooting above is a good general guideline.

Last Update: February 13, 2012