hi. I'd like to know what kind of hard drives are safe to low level format.

The kinds where you don't care if the data is lost. Or if you just don't want anyone else to be able to retrieve the data on the hard drive. The best way to low-format any hard drive is to download the utility from the manufacturer. These are free downloads. I suggest using the bootable floppy kind (or bootable image if you don't have a floppy drive).

A "low level format" is also known as a Zero-Fill of the HDD.

A "low level format" is also known as a Zero-Fill of the HDD.

But it not the same thing. A true low level format should only be attempted on MFM and RLL drives. The closest safe method of clearing or initializing a IDE/ATA drive is the zero fill option.

Seagate (http://www.seagate.com/support/kb/disc/faq/ata_llfmt_what.html) has a better explanation than I can give you...

I stand corrected, we tend to use the term low level format though.
low level formatting from Wikipedia:
Physical formatting, or low-level formatting, is the division of hard disk platters into tracks, sectors, and cylinders. Tracks, sectors, and cylinders define the divisions in which a hard disk accesses a data from a hard disk platter.

Hmmm... good to know...

we tend to use the term low level format though.

True. A lot of people still use the term "low level format" although it really isn't done any more, but the closest replacement is zero fill and it assumed the nickname. I guess since it was necessary from time to time with [much] earlier drive types, people just don't want to give up the term.