I scrapped an old hard drive from a old pc how do I clear all memory including the os? Thanks.
Burn gParted to a disk, and format it.
Hook it up with a spare sata cable and reformat it in my computer. Just right click on the new one and choose format.
If you want to really wipe it, you can use DBAN. When you delete a file, or delete the partition, the actual files are still there. Only the reference to where that file is on the hard drive is deleted. DBAN wipes the HDD by replacing all the 1s with 0s (Effectively making it like when you bought it). Also, I'm pretty sure it's good for an old hard drive to wake up some sectors. (It is a magnetic based technology, after all. Which wears off after time).
Otherwise, you can use any formatting tool which are pretty common. If you are planning to install an OS on it, most of them, like windows, offer a built in formatting tool.
DBAN ftw or ccleaner then do an advanced / complex / very complex overwrite.