Couldn't use a Leef USB 3 flash drive, neither for booting nor was it recognized by the os. Found a hint that the problem might be an incompatibility with the wifi card. After removing the card the drive works flawlessly.
Audiophile54, have you tried using another USB 3 device on the NUC?
Which BIOS version are you using? There is a new version available on Intel Download Center; however I would like to know which the current version is, in order to suggest you which version you could update to.