FASTBOOT randomly stops "seeing" my ZUK in bootloader mode

The title says it all.... the photo attached as well...
ADB devices command always seems my ZUK
when I boot into bootloader mode, my laptop starts dinging like a usb device
is plugging and unplugging randomly. Why is this happening?
Of course finally fastboot (or laptop) stops seeing my ZUK
and then guess, what? I get stuck in <WAITING FOR DEVICE>
Any ideas why is this happenig?

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