Lightroom and external drive workflow

So my iMac is 6 or 7 years old and I've been running Lightroom 2 forever on it so it's time for a new machine and to upgrade my software. As I'm looking to upgrade I'm considering a new Macbook Pro as I'm occasionally doing jobs where having a laptop will be nice to have. My collection of images is far larger than what would fit on the internal drive of the Macbook, so I will probably be having my catalogue on an external drive/raid (which of course would be backed up else where).

My question is: does anyone have a good workflow for importing images to the internal hard drive when not connected to the larger external, and then migrating them to the external drive later when it is connected again?

Is there any means of doing this automatically (eg: lightroom sees the external drive isn't present, saves the files locally, then next time Lightroom is running and sees the external drive, it moves them over there)? I have a decent idea of how I would do it manually, but being as I haven't actually done it, I wouldn't mind input of people who actually have to do this... Any pitfalls or problems? Any tips/tricks that make life easier? Is there another option (Aperture?) that is better at handling this kind of workflow?

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\~K