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I noticed that after cutting off power supply while the PC is running, some PCs can't be restarted for a minute or so. Why's that?

This is different when pressing the front power button for several seconds. Then, restarting works immediately.

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up vote 10 down vote accepted

Certain PSU's have a safety switch built in. This stops the PSU from being damaged when there is a sudden power failure, or a continues power dip that causes the power supply to switch on and off rapidly, which generally destroys the PSU.

I lost 2 older one's like this. Thankfully the newer one's will delay restarting long enough for the PSU to not "pop". This switch is only activated in the case of a sudden power failure. There is an actual patent for this you can read about here.

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For some hard disks there's also the matter of completely stopping the rotation before restart. – harrymc Oct 10 '09 at 9:22

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