Probably a BIOS (v3301) bug...(?)
I tried:
1) USB 3.1 and 2.0 ports, at the front of the pc, actually I tried all 4 ports
2) USB 3.1 at the back
3) 3 usb thumbdrives, 2GB, 8GB (brandnew, was still packed), 64GB
4) all FAT32
5) Inserted both after booting and prior booting.
6) also tried 8GB USB thumbdrive NTFS formatted using all ports at the front and USB 3.1 port at the back
Admittedly I did not try a USB 2.0 at the back (too much work getting there)
Note:
The usb thumbdrive(s) are showing up in the BIOS alright, as fs0.
When hitting F12 with no USB thumbdrive inserted, I am prompted to insert one.
After inserting the drive is detected, but F12 does not work.
(i.e. it seems to me that it is not really a hardware matter, the drive is detected)
Regretfully the manual does not provide additional information.
No word about fixing this F12 in the newer BIOS versions.
Asus X99 DeLuxe/U3.1 BIOS v3301
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later... updated BIOS to v3505, problem remains.
this time also tried USB 2.0 port at the back.
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