Mine was huge, and luckily I had an external disk with enough space to do that and it took about 5 hours for about 400GB of disk allocation. Just remember: when you create new image of an attached volume and chose not compressed but read/write, you need to have a disk with exact amount of space that totals the total space allocated within that name.dmg file (not the amount of space the data takes). Recovering all from thereon was a breeze. However, this time I was able to scan it with Disk Drill with exact file locations and folder hierarchy as I had.
If you follow online sources that tell you to perform first aid on your partitions using disk utility it will not solve your issue. This time this image was writable however, it still would not mount using Simonair's solution. After upgrading to Catalina, no dmg's are able to mount and throw a generic no mountable file systems. This time, I selected it and then clicked New image, and then selected image format read/write, not the compressed option. INSTEAD alternatively, I used Disk Utility, attached the Volume (yes I got the same message again, ''no mountable system files'' however, Disk Utility nevertheless attached the image, which I was able to do before anyways. I used the command from Terminal to convert but was not successful. I see that in this post some suggested that we should convert name.dmg to writable file.