Using dd to image a corrupted Windows disk?
Based on what I've done so far and the comments I've read on "Recover a dead hard drive using dd":


dd fails to image or clone damaged media to make the result equally mountable or readable, in multiple instances including mine;
there isn't an alternate tool in stock macOS that can do that.


In more detail:


The command I've been using in the question is the "proper" way - yet in many instances of working with damaged media, it does not create an image that like the original one, can be mounted and read. Basically, it fails at its task.
There isn't an alternative within stock macOS (such as ASR) that can clone or image damaged yet readable media. ASR (or its GUI sibling, Disk Utility) will bail out once there is an I/O error such as bad blocks on the media.
Haven't tried other tools that aren't part of macOS (such as ddrescue or dd_rescue mentioned in the linked article) - my answer is only about tools that are part of stock macOS.


The answer is based on multiple comments in "Recover a dead hard drive using dd" that mirror my experience: dd completes yet the image can't be mounted. I.e. dd does not appear to have created a close enough image of the media.

Here is what I have tried:


Use sudo dd if=/dev/rdiskX of=/Volumes/exFAT_Vol/image.dmg bs=4M conv=noerror,sync and sudo dd if=/dev/rdiskXsY of=/Volumes/exFAT_Vol/image.dmg bs=4M conv=noerror,sync to image the media. Neither image can be mounted, with errors such as "the following disks couldn't be opened: no mountable file systems".
Use the resultant image to restore to a (healthy) physical media via sudo dd if=/Volumes/exFAT_Vol/image.dmg of=/dev/rdiskZ bs=4M conv=noerror,sync - with a similar result: the physical media can't be mounted, and unlike the original damaged one, doesn't display the partition information such as "File system : Windows NT File System (NTFS)".
Attempt to use "First Aid" in "Disk Utility" to repair the images or the clones. None worked, with error messages such as:


Fixing damaged partition map. The operation couldn’t be completed. (com.apple.DiskManagement error -69874.) Operation failed….



What I haven't yet tried:


Use dd with a different bs= value such as 512, instead of 4m. Takes too long.
Use dd without the conv=sync option, i.e. just use conv=noerror one. (Just didn't have a chance to try it yet - each attempt takes 4-5 hours.)
use dd with conventional rather than raw devices - e.g. /dev/disk4 rather than /dev/rdisk4.
Cloning directly from the damaged media to a healthy physical one, bypassing the imaging stage.
Any other tools besides dd, Disk Utility, asr.


Hope this helps someone else in the same situation and really hope my answer is wrong: i.e. hope that there is a way to make dd work in this situation, to make mountable images off damaged yet readable / mountable media.