OK, someone - please remove the SOLVED thingy from thread title! I can now boot from NTFS flash drive in Compatibility (Legacy) Mode ONLY! However, this PC is fully UEFI and it boots perfectly and very fast in UEFI Mode. Every device is UEFI-compatible and Windows was installed in UEFI Mode too! What the hell is going on then? Should bootable drive be NTFS or FAT32?
Why don't you see if it boots using QEMU (e.g. RMPrepUSB F11) or VirtualBox+DavidB's VMUB utility? If it does not boot then something is wrong with the USB drive, if it does boot then something is wrong with your system/BIOS/settings.
I tried 3 different ISO's, 3 different USB ports (3.0 and 2.0), 3 different flash drives, 4-5 different boot media creators, clearing CMOS & resetting BIOS defaults, updating BIOS, but no matter what I do - I can only boot from flash drives in Legacy / Compatibility Mode... What could be the problem?!?!
OK - there is a special option in VBox for UEFI (Settings - System - Enable EFI (Special OSes only) and you need to specify Windows 64-bit x64 as the OS (if you specify a 32-bit Windows OS it will try to UEFI boot from \efi\boot\bootia32.efi).
The ISO you used for the USB wasn't UEFI. Make a UEFI ISO and format the flash drive in FTA32 only or use Rufus to create the UEFI bootable USB for installation.
Is your drive detected as a fixed drive or removable drive. Some Sandisk Extreme usb drives were designed as fixed drives to work with Windows2Go. From what I understand those are not bootable drives.
Shows VBox booted from Murphys AIO 24 version as given in link. I used Easy2Boot and made an image partition file from the ISO and added it to Easy2Boot. Then ran it under VBOX+VMUB in MBR mode, switched to .imgPTN and CSM menu, then selected the Clover CSM menu entry and booted in UEFI mode from the partition - no problems. I know it is in UEFI mode because it refused to install to my MBR partitions. I also booted in UEFI-mode from the same E2B drive under VBox and it did start to boot but got stuck at the splash screen. If you test your flash drive on VBox it should at least get to the Windows startup splash screen.
BAH! I got it figured out 100%. For some reason the flash drive would ONLY boot in UEFI Mode if it was formatted in FAT32 GPT format. It would not work with NTFS or FAT32 MBR format, but it is SUPPOSED TO. In fact, RUFUS allows you to select MBR format that should be visible by both Legacy and UEFI PC's, but not in this case, not even specific MBR FAT32 for UEFI would work. I got the OS, ALL updates, and all apps I need installed, registered, configured, etc. and backed up with Acronis True Image 2014, so if anything happens - I can get back to that near-perfect system state, where all I need to do is run a few small updates and re-install my games! Also, there is a new 355 update out there that came out for me 2x. First time I thought it was because I was messing with a language pack, but this time there was no language pack involved and the WIM already had Update 1 + all updates until July 2014 included, but the 355 update still showed up. I think its a pre-Update 2 file being released. I can't wait for more of that improved resource management and speed improvement in Windows 8.1 Update 2. BIG THANKS TO ALL!
So did it UEFI boot under VBox+DavidB's VMUB as FAT32? re. FAT32 formatting. Did you try UEFI booting both with and without an active partition? What if you change the partition ID from 0C to EF? Did your BIOS have a CSM+UEFI setting and a UEFI-only setting? Did you try both? Is there a BIOS update for your system?