Backups suddenly slowed to 181mb minute to SCSI Tape Drive

MS Server 2008 Standard x64 running Symantec BE 12 SP3. Backup up to a Dell 124T LTO3-60 tape library connect via Adaptec 29160-160U SCSI card.    One day backups are running at 900MB per minute to 1300MBPM. Next day they top out at 181MBPM. Drivers for 29160, Tape Library, and symantec's drive for tape drive have not changed. Tested with Dell for 5 hours, including using a dos utility that writes bypassing the backup software, still showed extreamly slow through put. They swapped out the tape drive, still same problem. Desperate for an answer to this problem.
adamant40Asked:
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Lee W, MVPTechnology and Business Process AdvisorCommented:
Completely remove, including drivers, the SCSI controller.  Then re-install.  LONG time ago (using Dell tape backup and Adaptec Controller - maybe even a 29160 - I had a similar problem and it turned out to be a controller driver issue.  We only found that out after completely changing out our backup software from ArcServe to Backup Exec and still having the same issue.
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adamant40Author Commented:
Thank you for your answer. Dell and Symantec verified we had the correct drivers. Symantec's driver for the tape drive itself. Microsoft Unknown library driver, and the 2007 adaptec driver for the 29160. Again, everything was working great one day, and slow the next w/ same drivers. Unless your suggesting that physically reseating the card might solve the problem?  
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Lee W, MVPTechnology and Business Process AdvisorCommented:
I remember my incident fairly well because it frustrated us for weeks and we did just about everything else we could think of.  Finally I think we replaced the card (and eventually reused it in another server where it performed just fine).  I just know it was the card that was the problem.  You may think you have the right drivers - and maybe you do.  And maybe it was a somehow changed registry setting... but whatever it was, it was definitely directly related to the card.

You may have a different issue, but I'd replace the card with an entirely different BRAND to ensure the same drivers aren't used... You can try reseating the card - it can't hurt... but I doubt that will work.  (When you have a weird problem like this, try EVERYTHING - so even though I doubt reseating will work, I'd say do it).
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DavidPresidentCommented:
Perhaps the tape drive is no longer operating at U160 speed, or automatically cranks down to U80 or even slower at times.  Unfortunately you can't rely on the BIOS or drivers or anything like that (But the BIOS can set the UPPER limit), because speed can change dynamically based on marginal signal or errors, then automatically sync up when it can.  This problem would be consistent with the symptoms, as it is automatically done per the SCSI spec.

santools  smartmon-ux will tell you interface speed and let you monitor it and report detailed communication & data errors. I am not aware of a freebie that will monitor interface speed.   You can always just try changing out the cable, adapter, terminators, etc.

Check the adaptec settings also, to make sure that the Upper speed is at the max. Do this at power on to get it at the BIOS

remember, speed can change on-the-fly, so it is difficult to catch this w/o special software.
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adamant40Author Commented:
Thank you all. I ended up shutting everything down and replacing the scsi cable and terminator. Restarted and backups ran at normal 1200 1400mbpm. I shut everything down again and put the old cable and terminator back on. Started up, ran backups, everything ran fine. No idea, just glad this is over.  ;-)
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