Seems perfectly alright. I think for HDDs, consensus is you overwrite everything and you're fine. If you want to make absolutely sure you can do multiple passes, like 2 or 3 with different (random) data should suffice. There are a lot of myths around though, concerning wiping data.

I generally use the common, established linux utilities: 'wipe' or 'shred' or just 'dd' on the whole device. The Arch Wiki has a long article on Securely wipe disk. I guess nwipe is fine, too.

Terms and conditions apply if you're using flash memory or SSDs. Overwriting them is not 100% effective. But for plain harddisks it is.