need help !

am Using a external firewire HD . all works well , capture to it direct , copy files from internal to external and back etc EXCEPT AVI files: copying them from internal to external is ok , but the other way round freezes de External HD . Same goes for trying to play them from the EXternal HD . it freezes too .
Any ideas would be greatfully appreciated
If more info needed please specify type of information needed

build in firewire in Sony Viao laptop !!
Filesize has no influence .

Thank you in advance