Microsoft Hyper-V Server 2012 R2: Setup asks for a product key

I spent the past 24 hours trying to install Microsoft Hyper-V Server 2012 R2. I tried to boot from a virtual CD containing the ISO downloaded from Microsoft, but setup kept asking for a product key.

After searching online and checking the physical server, I found a Windows Server 2012 R2 MSDN DVD in its optical drive. That additional installation media was causing the product-key prompt. I disabled the SATA optical drive in the BIOS and booted again. The installation then started as expected.

Check the installation volumes

1. Press Shift+F10 during setup to open a command prompt.
2. Run Diskpart.exe.
3. Run List Vol and review the attached volumes.

By this point, my problem was already resolved because the other installation media was no longer connected.