XPS8900 won't boot with external USB drive

Friend has an XPS 8900 with Windows 7 and UEFI BIOS mode.

When he connects an external USB drive (Seagate 5TB), the computer will not boot.  Instead it goes into a Dell diagnostics thing that starts testing his hardware components.

In a traditional BIOS I would just go into the BIOS setup and adjust the boot order. But with this in UEFI mode it shows "Windows Boot Manager" as the only boot option.

How can I tell the system to ignore the external drive and always boot off of the internal SATA drive?