Issues booting while a USB drive is connected is usually the result of a USB Device option being enabled in the System BIOS' Boot Order or Boot Sequence.
Some of these steps require accessing the system’s BIOS. Please refer to the system or motherboard manufacturer’s documentation for access and configuring options.
Here are some steps to try and resolve the issue:
Some System BIOS' may not include the ability to disable USB devices from the Boot Order/Boot Sequence. If this is the case, it is recommended that you disconnect the USB External Drive from your system until the system’s operating system has fully booted; at which time you can connect the USB Drive to your system.
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