E7450 won't boot from USB SSD

Due to the circumstances that I'm moving between multiple systems at different locations, I use an external USB SSD to boot into Windows 8.1. While this works perfectly fine, the boot fails whenever another external USB storage device (no O/S) is attached to the E7450.
In comparison, on a Latitude E7420 or a E7440 the boot always succeeds, regardless if any additional external storage device is attached or not. Only the E7450 fails with error "invalid partition table" and requires the detachment of any USB device.
The BIOS has been updated to the latest version and except for USB all other boot options were removed from the BIOS boot order list.
Any help or suggestion is highly appreciated.