I have successfully managed to get vhdboot to work on my surface rt, using a seperate external USB hard drive.
The annoying thing was trying to figure out why setup bombed out, but I got past that!
You'll want a brand new drive, or anything where you have nothing to lose, as you need to repartition and reformat the USB hard drive you want to use.
1) Follow this technet article: hxxp://technet.microsoft.com/en-GB/library/hh825686.aspx#RecommendedConfiguration - to set up the partition layout and assign drive letters to the partitions. Remember to update the partition sizes as suits your hd. You don't need to follow the part about the recovery image.
2) Create a new vhd on drive W: (going by the drive letters in that technet article!) and mount it. Use diskpart or computer management.
3) Prepare the install.wim of whatever build of Windows RT you wish to install (add your driver pack using dism). Remember that the build of Windows RT you want to use needs to be signed with the PRODUCTION certificates, NOT the test ones, otherwise secureboot won't let it run! (if you're using any public preview or RTM, then you don't need to worry as it's obviously signed with the prod certs)
4) Apply the new install.wim to the vhd using dism.
5) bcdboot W:\windows /s S:
6) With your initial Windows RT install running, insert the drive into your Surface RT. Boot from that usb drive (use a reboot, not holding volume down). It should go right into second stage setup.
7) When it reboots again, make sure to boot to your initial RT install, then reboot back into the USB. If you get a BSOD here, and you have rebooted into the usb drive from a running Windows RT, you might need to mount the SYSTEM hive of the VHD (Windows\System32\config\SYSTEM) and change HKLM\[mounted SYSTEM hive]\ControlSet001\services\FsDepends\Parameters\Virtu alDiskExpandOnMount from 1 to 4.
8) After rebooting successfully, third stage setup and oobe should go without a hitch.
After installation, remember you need to boot to your initial Windows RT install on C: and then reboot to the USB, not boot from USB by holding down volume down, or you'll most likely get a BSOD. I'm still not sure what causes the BSOD, and if you wanted you just install WinRT raw on the hd, instead of inside a vhd, there'd probably be no BSOD issues.
It's been a while since I last set up another VHD install on my surface RT, but if you have questions or problems reply and I'll do my best to help you!
The annoying thing was trying to figure out why setup bombed out, but I got past that!
You'll want a brand new drive, or anything where you have nothing to lose, as you need to repartition and reformat the USB hard drive you want to use.
1) Follow this technet article: hxxp://technet.microsoft.com/en-GB/library/hh825686.aspx#RecommendedConfiguration - to set up the partition layout and assign drive letters to the partitions. Remember to update the partition sizes as suits your hd. You don't need to follow the part about the recovery image.
2) Create a new vhd on drive W: (going by the drive letters in that technet article!) and mount it. Use diskpart or computer management.
3) Prepare the install.wim of whatever build of Windows RT you wish to install (add your driver pack using dism). Remember that the build of Windows RT you want to use needs to be signed with the PRODUCTION certificates, NOT the test ones, otherwise secureboot won't let it run! (if you're using any public preview or RTM, then you don't need to worry as it's obviously signed with the prod certs)
4) Apply the new install.wim to the vhd using dism.
5) bcdboot W:\windows /s S:
6) With your initial Windows RT install running, insert the drive into your Surface RT. Boot from that usb drive (use a reboot, not holding volume down). It should go right into second stage setup.
7) When it reboots again, make sure to boot to your initial RT install, then reboot back into the USB. If you get a BSOD here, and you have rebooted into the usb drive from a running Windows RT, you might need to mount the SYSTEM hive of the VHD (Windows\System32\config\SYSTEM) and change HKLM\[mounted SYSTEM hive]\ControlSet001\services\FsDepends\Parameters\Virtu alDiskExpandOnMount from 1 to 4.
8) After rebooting successfully, third stage setup and oobe should go without a hitch.
After installation, remember you need to boot to your initial Windows RT install on C: and then reboot to the USB, not boot from USB by holding down volume down, or you'll most likely get a BSOD. I'm still not sure what causes the BSOD, and if you wanted you just install WinRT raw on the hd, instead of inside a vhd, there'd probably be no BSOD issues.
It's been a while since I last set up another VHD install on my surface RT, but if you have questions or problems reply and I'll do my best to help you!
xda-developers