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I have a late 2009 iMac with the following hard drive: Capacity: 500.11 GB (500,107,862,016 bytes) Model: Hitachi HDT721050SLA360 Revision: ST0KA36E

This is happening nearly everyweek, usually overnight when there is nothing happening on it. When i come back in the morning the fan is spinning at over 6100 rpm. Normally it spins at around 1100 rpm.

What is causing this, and how can I fix it?

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Have you monitored the hard drive temperatures being reported? If so, what have they been? – Troggy Oct 27 '10 at 20:30
Yes, they are currently at 122(F) with the fan operating at normal speed of 1100 rpm. When it was spinning fast, it was down to around 110(F). – hwp08 Oct 27 '10 at 20:33
up vote 0 down vote accepted

Sounds like the system is doing some indexing / updating itself overnight and doing a lot of hard drive activity in the process.

Does the fan speed correct itself?

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No it does not correct it self, and there is minimal activity whenever it happens. There are no programs running when it happens. – hwp08 Oct 29 '10 at 13:03
How does the problem get corrected then? On reboot? Something you do in software? – Joshua Nov 1 '10 at 11:21

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