I have some news about this matter, which I came up with yesterday.
I appeared that I needed to see the partition, cause I wanted to write a boot record from BCD into that, so I had to assign a drive letter from DiskManagement. Afterwards, disk management does not allow you to "hide" the drive letter again, but DISKPART does. From within Windows, no boot-from-install-disk required.
So, to unassign the drive letter once you've assigned it, you must enter an elevated CMD and...:
Code:
DISKPART {enter}
DISKPART> list disk {enter}
disk 0
disk 1 etc.
DISKPART>select disk 0 (where "0" is your hard disk no. with the 200mb partition on it) {enter}
DISKPART> list partition (to find your 200mb partition) {enter}
DISKPART> select partition 1 (or your 200mb partition number.) {enter}
DISKPART> remove {enter}
DISKPART> exit {enter} The drive letter is removed and the partition is hidden again.