Sunday, March 18, 2018

Demon Diablo Disk Drive - attempted repair


A fellow Alto owner suffered a series of head crashes on her Diablo disk drives, which, in spite of careful cleaning or replacement of the heads, recurred. She had shipped the drive to us where we installed new heads and attempted alignment before returning the drive.

We encountered a series of unfortunate incidents during this process. Something was bent, undoubtedly due to shipping trauma, which led to bending a disk head irreparably.  The bad head caused minor crash damage on our precious alignment cartridge. The track pointer was indicating track 1 rather than 0 at the rest stop, leading us to misalign the heads by one track.

Finally, after repairing the bent parts in the drive, putting in new heads, resetting the track pointer and performing an alignment, we believed we had the drive ready to work properly. In order to test this, we performed a series of checks.

First, we used my disk driver tool to read one of our existing cartridges and verified that it read properly and got the track numbers right. Then we moved it over to the Alto and daisy-chained the drive to our working Diablo drive. With our drive powered down and the repaired drive set as the first drive, we attempted to boot up the Alto. It began okay but quickly reported errors.

We used Ken's ethernet tool, which has an Alto server inside with disk images, to rewrite the disk pack, but once again it quickly degraded. Running Scavenger against it, a program to attempt to repair disk corruption, resulted in crashes of the Scavenger program.

At points during the access we could see the software repeatedly restore the disk arm to track zero and seek back out again - a kind of error recovery action. During those excursions, it appears to randomly write or erase sectors as they come up with hard errors afterwards.

We tried to swap a board from a known working drive, but had the same behavior, random writing from software that should be read-only, seek excursions and corruption. By the end of the day, we only wanted to restore the images on the disk cartridges using our primary disk drive, after disconnecting the demon drive entirely. 

Of course, we opened and examined the cartridges for any sign of damage, plus inspected and cleaned the heads on all the drives. This is not a matter of crashing heads by moving damaged packs from drive to drive.

At the end of the work day, we found that we could not even write and boot from the images using our previously working primary drive. Something is going quite wrong. It is not software corruption since we reload the images. It is not head or pack damage. It acts as if some demonic hardware virus is jumping to the formerly good drive.

We could have damage caused to the Alto disk driver board as a result of some problem on the possessed drive in question - if that blew any driver chips then it might cause the good primary drive to cease operating. We might have had voltage spikes or other bad things occurring over the cable.

Remember that when we attached the demon drive, we had a cable from Alto to our good drive, a jumper cable from good drive to demon drive, and then a terminator on the demon drive. They were interconnected electrically. 

Next session we will check out power quality, look for problems on the Alto card and good drive, then debug the odd behavior of the demon drive. This is a battle to the death between that drive and us, but we are not giving up yet. 

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