Need Advice on External Hard Drive Error

I just purchased the OWC 750g External Firewire HD for my Imac G4, plugged it in and it appeared in my Finder. The User Guide says that I need to use Disc Utility to repartition the drive, which I did, in 3 partitions - two to be bootable drives for my iBook and iMac (160g each) and the 3rd to hold data from both computers (the remaining 378g). However, after completing this and hitting "partition" I got the 'Partition failed - Could not unmount disk' message. Any clue ... I'm not extensively techno savvy but I do my best!
Thanks!
Sorry - I'm new at posting, so this appeared a few times in the wrong place

Probably something as simple as a restart of the iMac would be enough to remind the iMac that it's okay to unmount the drive. 
Remember to use "Mac OS Extended ... (Journalled)" as the format for normal Mac use and as you have PowerPC Macs you should use "Apple Partition Map". Bear in mind you'd need to reformat for booting an Intel Mac.
cheers
mrtotes

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    4. To confirm that something is happening, open Activity Monitor (in the Utilities folder) and click "Disk Activity". Once you determine an average transfer rate, divide the size of the drive by that and then divide by 60 to see how many minutes it will take to finish. *+! ! FOR LARGE DRIVES THIS WILL TAKE A VERY LONG TIME ! !+* To be safe, do not use your computer for anything until the process finishes, you can leave Activity Monitor open so you can see when it's finished.
    If you're only really concerned about a certain files and you know their exact path on the bad drive, let me know and I can give you modified instructions to copy just those files, saving the time and space of doing the whole drive.

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