How to create a bootable partition on 4TB external hard drive?

I am replacing my previous backup drive, which I have divided in 2 partitions:

- an 8gb bootable partition that I can use for booting into programs like Macrium Reflect or even to install Windows from, or really any bootable media
- a data partitions for saving all my backups to

This way I can use this disk to backup my stuff, but also diagnose and reinstall systems or restore backups with rescue media.

I just upgraded to a 4TB disk and I would like to use it the same way, but I can't figure out how to make a bootable partition on it. On my old disk I would just make a 8gb partition and use Macrium Reflect or the Windows 7 USB tool to make it bootable. But on my new disk those programs don't recognize my partition.

I understand it has something to do with it being a large 2+ TB disk that needs GPT to be recognized. But more I couldn't find. Other manuals talk about installing Windows on it, but that is not my objective and want to just keep it as an external backup drive.

Win 7 64 can boot a GPT drive, but some boot able OS doesn't.
We want to see what you have in both drives.
Under windows, with both USB disks attached, open Disk manager, expand the columns so we can read them. Take a snapshot with the sniping tool of the whole window and post the image here as an attachment. Use the paper clip on the Quick Reply window then browse and upload the image.

I am not at my computer at the moment, so I cannot take a screenshot. But my 4TB disk is a GPT disk, with a simple 8GB NTFS volume and the rest is unallocated.

However if I start Macrium I see also a 128mb partition at the beginning of the disk that is not visible to disk management.

I have also tried the same setup, but with FAT32. In both setups Macrium and Windows USB tool don't recognize it as a USB drive and won't write the rescue disk or Windows to it.

Going to wait till you show us the two disks in disk Manager.
The 128 M partition is probably a MS$ reserved. Is it RAW or formatted (NTFS or Fat 32)?

I have formatted the boot partition in FAT32 and just extracted the bootable ISO to the partition. That worked. I can now boot from the external hard drive and access the my backups on the other 3+TB partition.

The only thing I am a bit worried about is that this hard disk cannot be used on non-UEFI computers. But I have no way to try it at the moment