Why does my system keep freezing while copying to an external HD?

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http://askubuntu.com – I am using Ubuntu 10.04 LTS (2.6.32-42-generic kernel) on my 2.1GHz Core 2 Duo laptop with 3GB RAM. I have a 1TB external harddisk that I have formatted as NTFS. Whenever I try copying a relatively (it has happened when the total size of the files was 4GB even) large number of files from my laptop, the whole system freezes: mouse doesn't move, clock stops, etc. Why does this happen? How can I fix this? (HowTos)