I'm guessing I won't be able to get much help with this since GPT and UEFI are so recent and presenting so many new problems and bugs, but I thought I'd make a thread anyways to get a conversation started and have something archived on the 'net regarding this issue.
I bought a ****** Dell laptop a while back, which came with Windows 8 and runs in UEFI (and SecureBoot, if that matters - I still don't know what the hell that is exactly). I set up a dual boot with Ubuntu 12.10 that took a while, but ended up working surprisingly well. However eventually I realized I was never using Ubuntu and booting into GRUB was unnecessary and annoying, so I ignorantly deleted the Ubuntu partitions without any thought to what that might do to my computer's booting capabilities.
Then my PC started giving me a number of different errors on startup depending on which boot options were selected - UEFI with SecureBoot, without SecureBoot, or Legacy boot. Booting in Legacy presents the text "Operation System not found", after which I CAN boot into a connected USB drive. UEFI without SB gives "error: no such partition. grub rescue>". As far as I can tell this prompt doesn't allow me to do anything, and I can't escape it to boot from USB. UEFI with SB gives "Image failed to verify with *ACCESS DENIED*. Press any key to continue." This repeats a few times before going to the boot options menu.
One final, important note about my available boot options: I can't attempt to boot anything but GRUB in UEFI mode! The options for a UEFI boot are Windows Boot Loader, Network, and ubuntu, and trying to boot to any of them gives the same grub rescue error.
For a long time I didn't recall that I had an UEFI setup, so I started researching an extremely similar error with the MBR: if one deletes Ubuntu partitions, their MBR still tried to boot to GRUB, which is no longer there, so it gives errors similar to mine. That error is fixed by using a Windows repair disc and repairing the MBR. However this fix does not work for me because, as I found out after many hours of frustration, my PC doesn't have an MBR in the first place; it's replaced by the ESP.
At this point I gave up on trying to save my files and decided to just do a clean install of Windows 8. I booted the USB image in Legacy mode (remember I am unable, as far as I can tell, to boot anything in UEFI without going straight to a grub rescue error) and deleted all partitions except my ESP.
Now the issue gets a bit convoluted.
From what I've read, my Windows 8 key/license is part of my ESP; I was never given an actual product key (I forsee a lot of crap-talking for using proprietary software - I'm sorry; I just love my video games!), and I have apparently no way of installing a licensed version of Windows after deleting my ESP, because that would delete my Windows key along with it.
So, as I said, I deleted everything but my ESP and tried to install Windows on a new partition, thinking that the Windows installation would do whatever it needed to do to repair the EFI and get rid of GRUB errors. Unfortunately, when I tried to select a partition for install, I received an error that read something like "Windows can't be installed on a GPT drive." After some research, I found out that this was due to my booting the installation in Legacy mode; to install Windows on a GPT, it must be booted in UEFI.
So basically, my problem is that I can't boot in UEFI to install Windows, but I can't delete the ESP either, because then I'll lose my Windows license. I imagine I could contact Dell and try to get some sort of key from them to do a new install, but I'm sure they'd give me a lot of ****, and there's no guarantee. If anyone has any thoughts, suggestions, or info on this issue, I'd greatly appreciate any input! I do need to do a bit more research on Windows 8 licensing in the meantime to make sure I have all my facts straight.