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I have a MacBook Pro with two internal drives: an SSD and a slow HDD.

On the HDD I keep heavy, rarely accessed files like movies, photos, etc. These are symlinked from the SSD, so applications like iPhoto and iTunes will still find everything where they expect.

I don't usually have the rotational drive mounted because it's loud and mostly unused. Is there a way to mount it when the system tries to access data behind a symlink, then unmount it automatically later?

(Intermediate *nix user, here, feel free to tell me I am asking for magic.)

Thanks for your help!

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Automatic unmounting might not be a good idea, since files are usually closed most of the time. Saving e.g. text files a few times will probably mount and unmount your drive all the time. – Daniel Beck Jan 1 '11 at 21:24
This would be my main concern as well. The utility would have to know when you are done with the file.... not an easy task when certain programs just load the contents into memory and don't place a lock on the file. – John T Jan 1 '11 at 21:45

You can configure auto mounting, but be warned that it's not for the faint of heart. It's too large a topic to just go into here, Google can tell you plenty about it. If you're feeling brave look at /etc/auto_home, /etc/auto_master and /etc/autofs.conf. That'll get you started.

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