he he gentoo...
One-pass zeroization: "dd if=/dev/zero of=/dev/xdy bs=ZZM count=WW"
x is h is pata, s if sata/scsi
y is device letter (a for first device on s/ata/scsi, b for second...z for 26th).
for blocksize, ZZ is your drive's speed.
Use the transfer rate from "hdparm -t --direct /dev/xdy" and round down the output a bit (i.e., my laptop's hd is 27.5MB/sec; I round down to 24)
A good idea is to enable 32-bit IO, dma, irq unmask, and write caching: "hdparm -c3d1u1W1 /dev/xdy"
WW (count) = (drive size in GB * 1000) / (transfer rate in MB/s)