With modern hard drives low level format means write 0's to the entire hard drive. On old hard drives low level format would mean actually changing some of the magnetic surface of the drive and restructuring it. Trying that with a modern drive would render it dead. As for the "full format" option, the only difference between that and the quick format is that the full format tries to check the surface of the disk for errors. Personal recommendations: I don't usually worry about doing a low level format unless I suspect a virus or other infection that I want to make sure is gone (i.e. old school boot sector virus) or if you want to keep your average user from browsing files (it won't keep determined people from accessing your data). If your reasoning behind doing a full format is to make sure your disk is healthy before doing a clean install, I recommend using the manufacturers diagnostic tool and/or Spinrite. The checking that is done during the full format really isn't all that great and I've had many drives that had bad sectors or massive amounts of read/write errors pass that. To sum it up: If your drive is suspect, there are more effective ways to take care of whatever your problem may be than trying to rely on the full format option. If your drive is not suspect, save yourself the time and just do a quick format. For the record, any used drive is suspect as far as I'm concerned until I check it out personally as I know it's possible for a drive to appear to be functioning just fine and still have a lot of problems and be near failure. Hope some of this helps you out.