Overwriting a 4096 byte sector harddisk drive with random data

[yudi v <yudi.tux@gmail.com>]

Hi

I need to write random data to a partition before encrypting it. Suggested way is to use urandom:

#dd if=/dev/urandom of=/dev/sda2

What is the use of operand "bs" in the following case? I see the above command executed as follows sometime:

#dd if=/dev/urandom of=/dev/sda2 bs=1M

For the hard drive I got, should I use the following command: 

#dd if=/dev/urandom of=/dev/sda2 bs=4096

From what  I understand, the first command above will write data in 512 byte blocks, the second one in 1MB blocks, and the third in 4096 byte blocks. Right?
I am a bit confused about the usage of this operand in this case. 

-- 
Kind regards,
Yudi