Cheers again Rob.
The HDD I'm using is a Western Digital WD5000AAKS - any specs I can find suggest it's R/W speed to be lower than what I'm getting anyway..
The cable I'm using is really old, and there is no writing on it to distinguish it - but the one that's running upstairs has "Gigabit CAT5e" printed on it so I'll drag that through and see if it's worth swapping them around.
512MB RAM on the NMT