Ask around if there's a Hard Drive degausser you could borrow.... Magnetic machine that will erase the HDD all at once completely.
Also, check your local shredding and data storage service providers. A lot of times, in addition to shredding info and offsite DLT backup storage, they will offer erasure services because they have their own degausser. (Especially if they offer computer recycling services)
Otherwise you'll have to put in the time and use software to write zeros for each drive one at a time.