Screen sometimes freezes when transferring to/from external hard drive

I have a brand-new macbook pro 15" (standard settings). Ever since I've taken it out of the box it's been having this intermittent problem where the screen freezes. The first time it happened was when I was migrating my files from my time machine backup. I didn't make much of it then but it has kept happening for the last 3 weeks or so since I got the computer.
When it happens I can still move the cursor around the screen but nothing responds. I can't close or quit anything, or even force quit. The only thing that works is to restart the computer using the power button. This has led to me losing work on more than one occasion (VERY frustrating). I've tried leaving it for a while to see if it eventually 'wakes up'. It doesn't. I've checked the hard drive and other hardware and haven't found any errors. I've installed all software updates.
All I've figured out is that this seems to happen when I'm transferring large amounts of data to or from an external hard drive. It happens with several different drives, which work fine on other computers, so it's not just the case of one malfunctioning external hard drive. It doesn't happen consistently either - sometimes transferring the same info later on works fine.
Has anyone had similar problems and/or (even better) found a solution? I'm planning to ring apple support once they're open and will update this if I find out anything else.

Tia,
Songs transfer to the iPod form iTunes. iTunes can obtain the song files from any location: main HD, secondary HD, ExHD.
You need to get the song files on the ExHD recognized by iTunes. Then you just need to sync the iPod to iTunes.
This is the high-level explanation. If you need more info, read the links below and/or post back.
iPod Tutorial
iTunes Tutorial

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