I have never used kfurge 1.0.7.8 firmware, but I think this has something to do with the filesystem that you use. It looks like you are using a journaling filesystem like ext3, where it needs to regularly update its jounaling data. Do you know what kind of filesystem you use?
If you are annoyed by the noise and don't care about the recoverability of the hard disk, you can just switch to a non journaling filesystem like ext2. Or if you use ext3, you can extend the "commit" time by adding "commit=<a number is second unit>" into your /etc/fstab. That is actually a quite generic configuration for Linux, so I am not sure if you could do that on your router with that firmware.
I hope that helps.
Anto