How can I safely create a partition to an already bootable drive I already asked this question on Apple's community forum, right here, so I'll post the answer here if something comes up there first.

This is my problem :

I recently bought an external drive of 500Gb with the intent to use it to store large files that I use on my Mac and to use it as a bootable drive for Windows.
I followed this tutorial to make the drive bootable on Windows 10, everything works well for that part.
Now I want to be able to partition the drive so that I can have 2 parts for Windows (1 part that contains the infos for the boot, 1 that contains Windows) and 1 part that I can use on the Mac side to store files. I thought that I could simple use the partition operation on the Disk Utility app, but the operation is greyed out. I found many answers on forums, but none of them is exactly what I'm looking for. Any idea how I can can achieve it ?

The actual state of the drive :

/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE          IDENTIFIER
   0:     FDisk_partition_scheme                        *500.1 GB   disk2
   1:                 DOS_FAT_32 WINBOOT                 209.7 MB   disk2s1
   2:               Windows_NTFS BOOTCAMP                499.9 GB   disk2s2


Thanks in advance and have a good day !
Ok, after spending the afternoon looking how to solve this, I found that the easy way was to create a new partition from Windows' side in exFat format. And since the format exFat is modifiable on Mac, unlike NTFS, I can then erase it and then change the format to whichever I want.