On-Call The working week's winding down once again and that means it's time for another edition of On-Call, The Register's Friday tech support tale recounted by readers.

This week, meet “Tim” who in the 1980s worked for Data General.

Yes, that Data General, the one that EMC acquired for US$1.1bn in 1999 so it could make hay with the CLARiiON iSCSI SANs (and quietly ignore its AviiON servers). Not many people know that Data General also kind-of almost invented the tablet computer, too. The company's device was called the Wiinpad.

But that's a story for another day. For now, let's get back to Tim's tale which involved “the maintenance of multi platter disk drives.”

“A lot of your readers may not be aware of these wondrous machines … but they were really something to behold,” Tim told The Register. “They were the size of a washing machine and contained as much as 50MB, plus a lot of scary moving parts.”

User dialled his PC into a permanent state of 'Brown Alert' READ MORE

“They came with a disk pack with 10 platters that rotated at 2,000 RPM. In addition there was a linear motor that moved the heads across the disk. Although it didn't move very fast it achieved the target speed with almost instantaneous acceleration and was a gigantic piece of moving machinery.”

This was in the 1980s, remember? So health and safety was yet to become pervasively paranoid. That meant that “these terrifying machines normally contained all their rotating parts within covers that protected the unwary from losing digits, however when being serviced you had to remove the covers and expose the whirling innards.”

Tim's story “started with a call from a solicitors saying that their disk drive was experiencing errors”. An engineer was duly dispatched and “quickly diagnosed that a fundamental part of the disk drive called an accelerometer was faulty.”

The accelerometer “told the linear motor how fast it was going and using negative feedback slowed it down or speeded it up as necessary.”

Accelerometers were easy to fix, Tom told us, but the tech who went out that day “also decided to check the alignment of the heads.” To do so, he used “an alignment pack - a special disk pack with a waveform recorded on its surface.”

“This alignment pack was very expensive and all engineers were told to take special care of it,” Tim pointed out in his mail to The Register.

Again, let's remember this was the 1980s and 50MB of disk was pretty wild stuff. So don't be too amazed at Tim's recollection that “We normally loaded the heads onto the alignment pack and then using an oscilloscope checked the heads were aligned correctly and adjusted as necessary using a micrometer.”

“This required your fingers to be close to an evil linear motor that could retract the heads off the disk in milliseconds and disk platters rotating at 2000 RPM. Somehow most of us kept our fingers.”

Eventually, all was in readiness. The accelerometer was working again. The alignment pack had installed. The on-site engineer was on the phone to Tim to tap his wisdom in case anything went wrong, so he pressed the On button.

And something duly went wrong.

“The drive spun up to speed and loaded the heads,” Tim said. Next, however, came “a massive explosion and silence.”

“There was then a long pause before a very shaken engineer said he would call me back.”

“It transpired he had made a very small mistake and connected the black and white wires from the accelerometer the wrong way round. The result of this was the negative feedback had become positive feedback and the linear motor was continuously told to speed up.”

“It smashed the 10 heads through the spinning disk pack releasing these as frisbees of death across the room, wrote off the alignment pack, all the heads and the motor. Somehow, miraculously, this happened without killing or injuring anyone.”

“I can still remember this incident with dread some 30 years later,” Tim concluded.

Have your tech support chores turned into near-death experiences? If so, write to On-Call to share your story.

On-Call has also foolishly signed up to do a talk on “Funny Open Source Sysadmin Stories” so feel free to fling those our way too! ®