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#140
You never had an issue with the other programs because those programs are more advanced, and Disk Management is for basic functions only.
A Basic MBR disk is only allowed up to 4 primary partitions or 3 primary partitions and 1 extended partition with up to 128 logical volumes in the extended partition.
When you try to create more than 4 primary partitions or create a primary partition in split partitions on a disk, then this is where the issue is. The 3rd party programs automatically use an advanced trick with a hidden created partition/volume to be able to do it. However, since Disk Management is only basic, it must convert the disk to be dynamic (not good for an OS drive) to be able to do this.
Of course, you need to know more about the ins and outs of disk management and partitions to know all of this before hand, but at least it does warn you first.