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My PC starts and runs from a SSD. Additionally there are to (spinning) HDDs in the system for data. I setup windows so that HDDs would spin down after 10 minutes idle time. This works great.

But during each system shut down, the HDDs spin up and then immediately spin down again, just before the system shuts off.

How can I avoid this?

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up vote 4 down vote accepted

you can't.

if the system shuts down it tells the apps to "get done with your job". some apps are then trying to "save" something to the disk, e.g. the "last opened document" or "this is the current state of the document the user worked on". don't forget: "services" (or "daemons" in unix-speak) are running in the background and are apps as well. they might want to flush something to the logs.

in addition to that the system has to detach the filesystem from the current session. for some filesystems thats easy (vfat), for some others (ntfs) a lot of magic happens behind the scenes. if the OS does not detach ("umount" in unix-speak) the filesystem properly the filesystem might be marked as dirty OR (even worse) gets corrupted. for that reason the disks are spinned up, the OS writes to the disk and then they are detached.

side note: you can detach a disk from the OS at any time (except the boot-disk) .. then it should stay idle. but if it is idle at the moment you detach it: it will spin up to detach properly, so you are only triggering the shutdown-spinup in advance :) read more about it:

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Thanks, that is comprehensible. Do you know if spinning up and spinning down immediately can harm the drive? –  Martin Mar 30 '12 at 8:08
    
well, it's mechanical. if you move things around they suffer. how much that affects the overall lifetime in measurable amounts? i don't know that. if you shutdown your machine once a day you will have 365+ spinups and 365+ spindowns per year. lifetime of a disk .. maybe 3 - 4 years, max? (lifetime as in "omg, i need more terabyte coz the disk has no more space left") ... –  akira Mar 30 '12 at 8:22

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