Macbook - AVCHD import - External disc drive - Import error

Question

Whilst attempting to import any AVCHD clip, I always receive a message informing me there has been an error during import and to check video carefully. Afterwards no imported video images are visible in the events list. i.e. the 'title' is there but no image(s). i.e. it looks like that the final part of the process which creates the small quicktime files has failed.
Now here's the interesting thing, this ONLY occurs when I am attempting to capture the video on my external 1Tb Iomega UltraMax disc drive.
It does not matter if I attempt the import as 'full' 1080i or the 1/4 sized 'reduced' import - both fail. Its just as if the processed output cannot be written fast enough. The import times are circa 4* the length of the clip.
When I however do the import to the built-in drive on the Mac, it works perfectly. Import time to the built-in drive is twice the clip length.
It does not matter if I attach the external drive by USB or firewire (though the latter appears to be slightly faster), both fail with the import. N.B. The drive is MAC formatted.
I have a 1 year old Intel Macbook (2Ghz & 2Gb, 120 Mb disc drive) with the latest version of Leopard and everything fully patched.
Has any one got any thoughts or solutions to this please?

Answer 1

Hi folks and special thanks to those whom put forward suggestions.
As mentioned in my original post, the external Iomega Ultramax 1Tb drive is MAC formatted, (and journaled).
One consideration was wether journaling was significantly slowing performance, but according to everything I read, there is little impact.
I took an old 60gb laptop drive and installed it into an external USB2 drive box and formatted as a MAC journaled drive. I then did an AVCHD import using iMovie 08 and all worked fine. I also carefully timed the import and the particular clip took exactly 59 seconds.
I deleted the import, rebooted, formatted the drive as a MAC NON journaled drive and did the same import. A worked fine and it took a fraction over 57 seconds. i.e. the overhead was just a few percent for journaling.
At this point it seemed to prove that something was very odd indeed going on with the Iomega Ultramax drive. This drive is used for time machine backups and time machine had never informed me of any inability to write to the disc.
Using the built-in Mac OSX disk utilities I ran a 'verify' on the drive. Guess what... it came-up with a number of errors including bad structures. I then ran a 'repair'. Unfortunately it was unable to resolve the the faults and I ended-up having to reformat the Iomega drive, as a MAC journaled device.
Tried a new AVCHD import to the Iomega Ultramax and it worked fine. In fact I later tried running Time machine (which was now doing a complete first time backup of my Macbook) _at the same time_ as an AVCHD import of many clips to the SAME drive. All worked absolutely fine.
For those interested in such things, memory usage during an AVCHD import was about 650Mb out of 2Gb RAM and processor usage (Intel dual core) was about 74% in each core.