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The DEC RP06 disk drive was a remarkable piece of hardware. It seems to have been the last generation of drives that used rigid magnetic disks in removable packs, before their replacement by 'Winchester' fully enclosed hard disks.

Unscheduled loss of power has always been a bane of disk drives; what happens if there is a power cut, or someone hits the emergency power off button, while in the middle of writing a file? But I've heard of worse; there was one story about one of the old drives – I forget which model, but one of the ones the size of washing machines – where someone tripping over the power cable caused a prompt head crash, physically destroying the head and disk. At the other extreme, I've also heard of drives where the rotational energy would be used to finish writing the current chunk of data.

So, to be specific: What happens to a DEC RP06 when there is unscheduled loss of power in the middle of writing?

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    Can't speak for RP06, but for IBM 3330 alike/compatible drives (which were developed way into the 1980s and sold into the 1990s), there was a power fail detection within the controller which tried to move all heads of data tracks with what's left in the PS. Keep in mind, disk PS do have rather large capacitors to level magnetic feed. – Raffzahn Nov 25 '20 at 22:47
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    > "It seems to have been the last generation of drives that used rigid magnetic disks in removable packs, before their replacement by 'Winchester' fully enclosed hard disks." The last DEC drive with a removable disk pack, was the RA60. > "At the other extreme, I've also heard of drives where the rotational energy would be used to finish writing the current chunk of data." This was done by the DEC RA90 disk drive. – fatdoor Nov 27 '20 at 15:04
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The RP06 maintenance manual says

Emergency Retract. An emergency retract operation is used to remove the heads from the pack if the retract operation fails, or if there is a power or power supply failure.

and

3.4.2.3 Emergency Retract Mode

Only one positioning operation can occur in this mode, this being an emergency-retract operation. In the emergency-retract operation/mode, the heads are unloaded and retracted from the pack whenever a serious malfunction warrants such action. Examples are logic voltage failure, retract failure, overcurrent failure, and supply voltage failure. During this operation, heads are cammed away from the disc surfaces as the heads move in reverse out of the pack.

So, it seems that when loss of power is detected, the heads are immediately retracted off the disc by logic in the drive unit. I don't believe the current write, if any, is completed, but I did not find specific statement of that. However, all mentions of emergency retract suggest it's immediately triggered by loss of power (and by other fault situations).

The power supply seems to have arrangements for supplying "emergency retract power".

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    I wonder if this was an electrical thing - full positive voltage to the actuators or something - or a mechanical thing - some solenoid lets go and some retractor is freed, being powered by the energy left in the still spinning disk pack ...? – davidbak Nov 26 '20 at 2:43
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    It is probably a combination of mechanics and electronics: the spinning platter actually creates the pressure to make the head float. The larger drives from past times had probably much more inertia to sustain this longer. – chthon Nov 26 '20 at 8:31
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    @davidbak. I had some old Control Data 'washing machines'. The emergency retract consisted of a relay and a big capacitor. When that relay dropped out the capacitor was connected to the voice coil and the heads retracted quick sticks - you didn't want your hands in the way as you would've lost fingers. As well, the drives didn't have any 'intelligence' - the controller told it to enable the write circuits and sent it the write data bit by bit - just like the ST506 drives that came years later. – Kartman Nov 26 '20 at 8:46
  • @Kartman The fact that pressing the emergency stop makes things move at high-speed is a bit scary. – user253751 Dec 8 '20 at 16:30
  • The loud thunk was preferable to the sksssssh of a head crash. Besides, everything was enclosed. You needed to be careful if you worked on the innards though – Kartman Dec 9 '20 at 5:12

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