How can I fully backup Windows, Linux and various boot partitions of an external drive?

I have an external drive for my 2017 iMac with a Windows-to-go and Linux system installed on it. It took a lot of work to get this set up just the way I needed, and am trying to find a way to make regular backups of the drive so that I can restore either system if things go wrong, especially given that Windows-to-go has no recovery environment. This means I need to include their boot partitions in the backup as well. There's both the EFI system partition (as this drive has a GPT) as well as a separate boot partition for Linux.

Furthermore, I'm hoping that however I decide to backup these drives can be included in the online backup service that I use. (i.e. CrashPlan or Backblaze)

I'm not sure what the best way to go about doing this is.

So far I've dumped the raw contents of each partition to a separate backup drive using dd in macOS, e.g.:

dd if=/dev/rdisk2s2 of=/Volumes/Backups/external_drive_partition_2 -bs 1m

...and that works. I've already used it to restore my Windows partition after accidentally blowing it away.

The trouble is that these partitions are 200 GB each, and I can't have even one 200 GB file that regularly changes and back it up with an online backup service. It would take a week or two for it to upload just once with my internet connection.

My only idea so far is to do something like the dd solution, but break up the file into multiple parts, say 50 MB a piece. Then when making the backup, somehow only update the pieces that have changed since the last backup. That way, only those files will be uploaded to my online backup service. Short of writing my own software, though, I'm not aware of a way to do that.

I'm open to other ideas and software recommendations. My main operating system is macOS so if I were only going to have to backups be made on one OS, it would be that, but if there's a way to do it in either Windows or Linux I'm open to that.

Answer

After some time, I eventually decided that the method I outlined was probably the best way to do this. So I wrote a piece of software that allows me to make backups the way I want: multipart-backup.

Perhaps someone else will eventually find it useful too!