Windows 10 is not yet ready for the masses. Microsoft hasn't confirmed rumors that it the new operating system will be ready in July in time for back-to-school PC shopping.

Still, if you want to try an unfinished version of Windows 10 right now on a Windows 7 or 8 PC, you can, through the Windows Insider program. And 3.7 million people have already done so, Microsoft says.

But there's one group of people who can't: The ones using Microsoft's brand new, low-cost Surface 3 tablet, which went on sale last week.

In answering questions about how to upgrade the new tablet on Microsoft's Surface forum, a support person warned:

Please do NOT try to install Windows 10 on the new Surface 3

There are no drivers in the Preview Build because Intel has not yet provided drivers.

There are no drivers for the Intel x5/x7 Atom processors

This warning does not apply to the new high-end Surface Pro 3 tablet, which can handle Windows 10 just fine, the Microsoft support person says says.

This half explains one Microsoft mystery.

At its Build Developer's conference held in San Francisco last month, the company gave attendees a new HP Spectre 360. Most of these attendees, Microsoft developers, are part of the Windows Insider program and immediately upgraded the tablet to Windows 10 Preview.

But attendees were baffled why Microsoft didn't give away one of the new Surface tablets, such as the Surface 3. Now we know why.