I would recommend a different USB drive. I have had similar experiences booting from a Corsair USB drive where it would disconnect itself soon as the laptop POSTs unless I choose Boot from USB and then quickly remove and re-attach it. Luckily the Voyager series USBs have a light indicator when it's online which helped with the timing. if no other drives are available try this.
We bought a Dell OptiPlex 5250 with the latest chipset (Core i5 Gen. 7) lately. I started to create a bootable USB stick with DIA but after booting from this stick(with UEFI, but also with legacy) it says:  Initializing Dell PE, this may take a minute... Waiting for USB Drive to be ready... Unable to locate USB Drive. X:\windows\system32>   I found no way to boot from the DIA-USB stick, neither it is located in the 3.0 nor the 2.0 USB ports. Any suggestions, what I did wrong?
USB drive not detected