Same problem here with an Intel Atom NUC, using an USB stick (Flash update fails).
Finally, I use an old USB stick (Kingston Data Traveler 2GB, FAT format) and the update succeed, so I think there's a problem with new USB sticks and the file system format used.
I have seen what I think is a related problem with some UEFI BIOS (including Intel NUCs), reading the EFI partition which must be in FAT32. Sometimes, if you don't use a specific cluster size, the BIOS can't read the EFI partition.
This link from Microsoft describes the default cluster size for FAT32 formatted disks, check against your USB stick:
http://support.microsoft.com/kb/192322
- Davor