Windows 7: Clone defective disc and then securely wipe

My Dell hard disc is failing according to SMART so I am going to get free replacement from Dell and they will take old one.
I have already imaged the system using DriveImage XML ignoring "i/o device errors". But I doubt I can restore this image.
Can I directly clone the hard disc with Raw Copy? Will that work?
Before returning my hdd to Dell I want to make sure to delete it securely. What is fastest way. If I just delete the partitions, make new ones of different size and format to Linux ext4, will that wipe my old data securely?

Dell will safely destroy the old hdd and should be returned with an operating system installed, then when you get it returned make a system image backup, keep it in a safe place, preferably on an externall hdd for future use. Don't use old backup's.

Actually the Dell technician is going to do everything on site. I want to clone the defective disc to new one and securely erase it before giving it to Dell.

That could void your warrantry.

To have a full "clone" of your existing HDD use free Macrium Reflect and image EVERY partition to an external HDD.

As stated securely erasing all your HDD may get you into warranty issues.

If you have sensitive or important personal information on the HDD also manually copy to the external HDD and delete it.

Check with Dell to see if they plan to take possession of the old HDD. If so, free CCleaner has a utility to write zeroes to deleted files to ensure they cannot be recovered. I am not suggesting you use CCleaner for any other purpose.
Edit: To be specific in ccleaner.
Tools>Drive Wiper> Free space only - I would only bother with 1 pass.