Creating a compressed image of an exFAT USB drive using dd

My system is running Ubuntu 22.04.3 LTS. I plug a USB drive into that system. That USB drive (/dev/sdb) contains an Ubuntu installation, mostly in an ext4 partition (/dev/sdb3). That installation fills about 25GB of drive space. With /dev/sdb3 unmounted, I use sudo zerofree -v /dev/sdb3 to zero free space on that USB drive. Then I use the following command to create a compressed image: sudo dd if=/dev/sdb bs=16M conv=sync,noerror | pv | sudo pigz -c > /media/TargetDrive/UbuntuImage.dd.gz This seems effective. The resulting .gz file weighs only 12GB. Now I remove that USB drive (/dev/sdb) and replace it with a USB drive containing a YUMI exFAT installation. When I seek information on this /dev/sdb drive using GParted, I get a warning - "Unable to read the contents of this file system!" - and an indication that exfatprogs is required for exfat file system support. I install exfatprogs and restart GParted. That eliminates that warning. This YUMI exFAT installation fills 43GB on the USB drive. I understand that zerofree and Microsoft's sdelete -z E: don't zerofill on exFAT, so I follow advice that seems to recommend the following: sudo mount -o rw /dev/sdbX /mnt sudo dd if=/dev/zero of=/mnt/zero_file bs=32M sudo sync sudo rm /mnt/zero_file sudo umount /dev/sdbX Those commands duly produce and then remove a zero_file whose size does appear to fill empty space. (I saw but did not try this alternative, which was less clear to me: cat > /dev/sdbX/zeros < /dev/zero ; sync ; rm /dev/sdbX/zeros.) Then I run substantially the dd command shown above. This time, dd and pigz do not achieve comparable compression. The 43GB exFAT installation is saved in an image file of 42GB. Granted, the exFAT drive's contents are produced by Windows. But experience with Windows drive imaging software indicates that I should expect substantial compression. My question: can the dd command be improved to yield better compression for this exFAT drive? Note: I seem to have obtained similarly poor compression with an NTFS USB drive, but did not document each step in that case.