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Computers support hotswapping so that a user can replace a harddrive while the system is running. Is that mostly software just powering off the harddrive or is it some special hardware involved? If there were not special hardware then I suppose that you could hotswap any drive but it seems that the drive must support it in hardware.

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    \$\begingroup\$ Notice how the connections on the hot-swappable devices are different lengths. They will be disconnected at different times when the connector is disconnected. So the software notices a hardware change and says "Uh-oh!" \$\endgroup\$ – Andrew Morton Jan 14 '17 at 22:41
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It requires several features, some of which put the cost up.

These include additional transient suppression on signals, which may put additional capacitance on bus signals thus requiring additional engineering/testing and may reduce performance below what's otherwise possible.

Also, connectors which longer and shorter pins that ensure that some pins mate before others and disconnect after others. This is to guarantee that signals are connected and disconnected while power and ground are already connected. In some cases, a control pin can be made to connect last and disconnect first, and this pin will tri-state all other signal pins.

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Reliable production level hot swap typically requires both software and hardware support.
You may need software to ensure the device is free'd so that hotswap can occur. The software needs to be able to integrate (size, format, initialize, rebuild) the new device back into the environment. For example if you lose a drive in a redundant array, you need to be able to rebuild the array after changing a drive.
You may need hardware support to turn off the power within a chassis to remove and replace drives.

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Modern hot swappable cards usually have a hotswap controller to limit the inrush current when a new card is plugged in. The issue is simply that when a card is plugged in, any bulk capacitance must be charged up, which can cause system voltages to droop; this issue is then exacerbated by the fact that most cards have switchmode power supplies.

These can best be understood as true power converters (to a first approximation, at least). If the system power rail(s) droop, then to deliver the power the card circuitry needs, the input current must increase, adding to the load current on the system power.

This can cause brownout if not properly dealt with.

Techniques have evolved over the years for these applications.

As already noted, there are special connector arrangements for hot swappable cards and in many cases system software needs to manage the event.

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