From an environmental standpoint, it's undesirable to use a physical DVD for installing an operating system. It's also not necessary. Instead of installing Windows Server 2008 R2 beta from a DVD, I used a USB flash drive.
A USB flash drive can be made into a substitute for a DVD in just three easy steps without using DISKPART commands.
1. Attach the USB drive. Format it as NTFS.
2. Mount your iso file using MagicDisc, Daemon Tools, or Virtual CloneDrive (my recommendation).
3. At an elevated command prompt, enter the following:
xcopy d:\*.* /s /e /h /f g:\
D is the drive letter for your mounted iso file (this could also be a physical optical drive with an actual DVD)
G is the drive letter for your USB flash drive.
If your drive letter mappings are different, make the appropriate substitutions.
XCOPY switches are described here.
4. Attach your USB drive to the machine in question and boot. You may have to go into setup to force the machine to boot from the USB device. You may also have to enable booting from USB in the BIOS.
In addition to using USB flash drives, you can use laptop drives attached via a USB adapter or external hard drive case. Not only will you save the environment, you will also save time because optical devices are relatively slow. I typically have old laptop drives preformatted and ready for an XCOPY.