How long does low level format of external HDD take

The terminology "Low Level Format" as used by MHDD32 and everyone here is incorrect. MHDD32 uses this term to refer to issuing an "erase" command to every sector on the hard drive. This is NOT what a true "Low Level Format" is. Writing to every sector will force the drive's firmware to indeed "recalculate ECC" and also remap bad sectors into spare sectors (if it has any left) - so this is useful for repairing drives with bad sectors (although the disk should really be replaced soon).

A low-level format is a process where the drive writes things like sync marks and sector markers to the disk, which would only not be present if the disk is basically a fresh unmagnetized platter from the factory. A disk that has not been nor ever been truly low-level formatted will therefore be physically unreadable. Writing all zeros to all sectors, or any sectors, requires the basic "framework" signaling be written on the disk for the drive controller to be able to find specific sectors.

There was a time in the early to mid 80's before Windows dominance where floppy disks were typically sold "unformatted" - and this low-level format had to be performed for the disk to be useable. The old DOS FORMAT command did the low-level format automatically unless you used the /Q switch for a "quick format" - which was not possible on a never-formatted disk. Linux has separate commands for the low-level (fdformat) and high-level (mke2fs, etc.) format.

The old MFM and RLL hard drives were the same way, but they typically came pre-low-level-formatted from the factory (complete with a sticker listing known defective sectors). But you could perform a true low-level format - one way was to call the low-level format in the controller's ROM directly using the old DOS DEBUG command (http://discussions.virtualdr.com/archive/index.php/t-69312.html). Also read this: http://www.redhill.net.au/d/10.php

IDE drives did not typically provide a user accessible low-level format routine or command and therefore it's not possible to do a true low-level format on these types of drives. You never know what types of diagnostic or debug commands are available on a drive, and what the code on the firmware is capable of doing, but that is the only way a true low-level format would be possible on modern drives.

Boot from a Linux LiveCD/USB and try GParted (or perhaps from GParted's own Debian-based LiveCD/USB).

If that doesn't work, see if you can take the drive out of the case (obviously don't do it if it'll void the warranty) and directly attach it to a desktop so you can run GParted on it.

On Gparted you'll have to "Create a new Partition Table" and then create any partition you'd like because it's going to format for it.

If you still get I/O errors or GParted fails, use WinDFT and run an Extended/Thorough Test on the drive. If that passes, Erase the Disk and attempt to repartition: