I do actually have df on my developer version empeg (which you'll need anyway) - it came with it, I didn't add it. I think it used to work. However it now says "df: cannot read table of mounted filesystems: No such file or directory". Dunno what that means exactly.

I figure I'll worry about it when disc space starts getting low. I know I have about 20Gb free but I don't know how it's spread across the drives

Edit : It appears to be something to do with /etc/fstab or possibly a non existent /etc/mtab.