How does the shredder work?

Hi,

I've changed my main data drive from a 2Tb to a 3TB, however there is a problem with the 3TB which means it's going very slowly at times (I think it's due to a lot of bad sectors and it has failed the manufacturers diagnostic tests) as such I'm sending this back for a replacement. I'm therefore shredding the content of the disk first.

I started this on a 160GB partition that contained about 80GB of data, using the American Standard DoD method this has so far taken 18 hours, the progress bar is just under half way across and there is about 34Gb of data left on the disk. Now rough calculations tell me the whole 3TB should take about 3 days to complete if the drive was running along at a healthy 40MB/s, however currently it is going at 200KB/s, so if this continues that's 2 years to complete the job (it it completely dies, which ever happens first). At which point it is more economical to use a hammer and a screw driver on the disk and buy a new drive instead of waiting for the shredding to finish and trying to send it back after this time.

So here comes the questions:
1) Given I have selected a partition to shred, will this shred:
a)just the non-deleted files,
b)the non-deleted files and areas that have previously held files,
c) all of the partition including empty space that has never been used?
2) The multi-pass methods, how do they work, do they do:
a) a complete pass of the whole data before starting the next pass,
b) a pass per file then the next pass on the same file, or
c) three passes over each sector before moving to the next sector?
3) If I do a simple pass over the data to change it all to zeros, how easy is it to extract the data back, as I'm only interested in stopping opportunist criminals, not a dedicated target from criminals or security forces.
4)Is there anyway I can get the shredder to not attempt multiple reads/writes on sectors and instead log the file that it is part of and still shred the rest of the file, so that the vast majority of the healthy drive can be wiped relatively quickly, only leaving the files with bad sectors behind to be dealt with separately (e.g. if they are just program files leave them un-shredded, if they are data files then use a weaker/faster process on them)?
5) what is a good program to look to see what is left on the drive after shredding, namely files that have been marked as deleted but not shred I'm guessing.

Basically I was hoping to just shred everything and forget about it, rather than to have to worry about which bits of data actually need shredding and which don't.

Thanks,
Best Regards, Gary