Sorry, I did that twice.
It still occurs every time I 'safely remove' the external hard drive. This has only occurred with the lucid kernel - prior kernels (going back as far as Ibex) did not have this issue and the hard drive partitions and filesystem types are unchanged. I will test the current development kernel tomorrow and let you know. I unfortunately don't have the bandwidth available to download a daily build - I'm on very limited bandwidth at present. I apologise.