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I have a weird problem with an external disk connected to my 27" iMac running OS X 10.6.5.

It is a 1TB LaCie d2 Quadra and it is connected through Firewire.

I use this disk primarily for Time Machine backups, which works well and fast.

The problem is that when this disk wakes up, it locks up the complete iMac. While I hear the disk spin up my iMac is unresponsive. Then when the disk is ready, the iMac comes back to life.

What is going on here?

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What is going on is that OS X (actually it’s the Finder and the I/O subsystem) is “waiting” for the drive to be readable.

Truth is, is not the “whole mac” but the Finder (and any Finder application) what freezes. For example, a Save/Open dialog, Time Machine starting, a Finder window, etc. However you can always cmd-tab and perform other actions.

There isn’t much you can do, except to tell OS X not to sleep your hard drives (or wait for the drive to spin up):

Energy Pane. Beware with that option, because it will mean that your hard drives are on all the time, thus degrading their lifespan.

I’m not 100% sure if that option affects Firewire/USB external drives, but I think it does.

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I think you're right, and I think it happens with both USB and Firewire type drives. I have an external RAID 1 FW800 setup and every time the drive goes to sleep and the finder kicks in (or an app brings up a file dialog), I get a 15-20 second pause while the 2 external drives spin back up. –  robsoft Sep 20 '10 at 11:07
    
One can only hope we’ll soon get SSDs everywhere :) –  Martín Marconcini Sep 20 '10 at 11:23
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