trouble formatting 3TB Seagate external HDrives. need help

[paul condon <pecondon@mesanetworks.net>]

I have two 3TB Seagate external HDs. They were purchased from different 
stores at slightly different times earlier this year, here in Colorado. 
I want them to have ext4 file systems on them, excepting if someone on 
this list can give a reason otherwise. I have googled and gotten a lot 
of hits, which indicate to me that this is a well known problem. 
Unfortunately, I have difficulty following the instructions, and all my 
efforts have not reached a successful conclusion. Now with further trys, 
it seems to me that stuff has been written onto the drives that needs to 
be wiped off because I get messages that from the disk utility in xfce4 
that it won't overwrite a disk with data on it.

So I want to use dd to wipe a complete drive.
For this I have found the following:
# dd if=/dev/zero of=/dev/sdg bs=1M
In the above, I have already changed the HD device to sdg (from sda), 
but I wonder about bs=1M. Could the process go faster with a larger 
block size? What are the criteria for choosing a value for bs? And, how 
long should a 3T wipe take to complete? The job has been running for 
about 12 hours. Would it go faster with a different bs? Faster enough to 
make the waste of 12hrs running worthwhile? Is there some way to invoke 
an 'progress indicator' for dd? And, in general, is there a better way?

TIA
paul