How to securely wipe your ReadyNAS

Update:
I realized that I was going to spend many hours or days waiting for srm to securely wipe several hundred GB of media files that I don't really care about. So I decided to abort that process after 6 hours. The files I care about are already deleted and just need to be wiped somehow. Of course there are many theories and opinions about this... but the idea of just overwriting the data blocks by filling the drive space with zeros makes sense to me. And I am not some secret government body with classified secrets to hide. I just don't want some stranger to be able to recover my personal info, photos, or passwords.

I just came across the following suggestion as a way to write zero filled files to every free block on the disk and then delete them:

dd if=/dev/zero of=hugefile; sync; rm hugefile; sync

So I could probably be happy with something like that... but then I noticed that the OP of this thread already addressed the free space question with the sfill command.

sfill -v /c/

That's running now with the full 38 passes using random data, so I expect that could run a while.
When that's done I believe I will run it a second time with the -l switch for 2 more passes... just for fun.

sfill -l -v /c/

Then I will do the factory reset. I think after that I'll feel pretty good about it without having to plug each drive into a PC and reformat etc.
And if I can sell each drive separately all the better. I will probably keep at least one for myself.

FYI - Here is some additional reading I came across on the subject of dd and sfill in case anyone is interested:
https://bbs.archlinux.org/viewtopic.php?id=83672
http://www.noah.org/wiki/Dd_-_Destroyer_of_Disks
http://www.ubuntugeek.com/tools-to-delete-files-securely-in-ubuntu-linux.html

I really appreciate the input mdgm and of course many thanks to Tristan for the detailed instructions.