Well, the title of my question pretty much sums it up, but let me elaborate. The main idea of what I want to do is as follows:
1) I have two separate hard disks, with nearly equal performance. I use them as "basic" discs (contrast to "windows dynamic disks"), and intend to keep them that way for numerous reasons.
2) However, sometimes I want to use some advantages of Software RAID, that dynamic disks offer.
3) Windows 7, after just some 15 years now, finally caught up with Linux (and its "loop devices") and introduced a notion of "virtual hard disk", which is really a file on a real hard disk.
4) So, I wanted to create two virtual hard disk files on my two different hard disks, make them dynamic, format them, and finally join into one, for example, striped volume (aka RAID 0). This way my real hard drives stay "basic", and I get all advantages of "dynamic".
It looked like a nice idea to me, with no indication, that it might not work, yet it failed. Turns out, even though you can make dynamic virtual hard drives, you can't join them into RAID for no apparent reason (relevant menu items in Disk Management are grayed out).
Microsoft says
You cannot configure two attached VHDs to be a dynamically expanding VHD. A dynamically expanding VHD is a physical disk that you have initialized for dynamic storage. It contains dynamic volumes such as simple, spanned, striped, or mirrored volumes or RAID-5 volumes.
i.e "It can't be done, because it can't be done, so get lost now."
I guess, I can find another ways to achieve what I want (i.e software raid without making my discs dynamic). But I want to make it without using third-party tools and drivers, in a fully "system-supported" way.
Therefore, the question is:
Does anyone know some kind of not-very-dirty hack to make it work?
Best Answer
I convert my comment to an answer, as the comment by @C0D3M0NK3Y confirms it.
Windows will not accept dynamic virtual disks in a RAID, but the comment by @C0D3M0NK3Y confirms that fixed-size disks used to work in XP Professional. There is therefore an excellent chance that fixed-size virtual disks will still work with RAID in Windows 7.
It is actually quite logical that Windows cannot create a RAID over dynamic disks whose size is unknown and can change at any moment.
EDIT/ADD:
Had to try it to see either way for sure, and it works (when using fixed-sized VHDs):
Host OS: Windows 7 Ultimate, with two "Basic" disks.
VM OS: Vista Ultimate.
Created two "fixed sized" VHD's of 500MB each, one on each of the host's physical disks, and attached them to the VM (as the 2nd and 3rd drives).
Initialized the new disks in the VM and then converted them to "Dynamic" disks (in Vista's Disk Management).
Then joined them together into a RAID0 (stripe):
Worked like a charm, without any need to do anything 'weird'.