You haven't said why you want to RMA the drive, but for
some faults then any attempt to erase the drive may be unsuccessful.
To answer your earlier questions:
bill4d wrote:
I know that I only had about 125 GB of data on the disk.
Would I be safe to run the erase script up until it reaches 125 GB ?
No, because...
bill4d wrote:
When a new hard drive is written to, does it start writing on the first sector and then second and so on?
That's actually a question about the filesystem, not about the drive itself, and for typical PC filesystems the answer is "no", which is why you can't just erase (overwrite) the first 125GB and expect to prevent access to your data. Filesystem algorithms are typically (much!) more complex than that, especially if there has been much creation & deletion of files.
I don't currently have time to write even more detailed instructions, but the "internal security erase" suggested by
BlackST is more commonly known as Secure Erase (although the (S)ATA command has a slightly different name). There are several different techniques / utilities to (try to) invoke a Secure Erase. Since you're not experienced in this procedure, IMHO the two easiest utilities which I can think of to do this process for you, are to use
MHDD in a specific way, not the way you're trying at the moment (note the author of this link

):
errata-mhdd-fasterase-t9867.htmland
HDDerase:
http://cmrr.ucsd.edu/people/Hughes/SecureErase.shtmlBoth of them are DOS programs which (typically) require the PC's SATA controller to be put (temporarily) into IDE / Legacy / Compatibility mode (whatever your BIOS calls it). I'm guessing you've already done this but, in my experience, HDDerase can be more fussy than MHDD. Remove all drives except the one you are trying to erase.
Hopefully this is enough info to get you started. Good luck but, as I said, there is no guarantee of success, especially for a faulty drive.