In some cases, the USB Drive shows up as only 0-bytes long, so you cannot put anything on it. That just means that it is not exported correctly at that stage of booting. If it crashed later in the boot process, the full USB Drive and its contents would be visible.
If you boot far enough to have access to the USB Drive, you should be able to use data.tar.gz to examine and repair it.
On my k4nt, it is stuck in USB HID mode because part of the MMC flash storage got somehow corrupted, so it no longer has a valid serial number and board ID. The u-boot bootloader says that it cannot initialize the hardware for an unknown board type, so it drops into USB HID mode.
There is work in progress by devs to figure out how repair a kindle using USB HID mode.
So there is hope. You may want to hold off on discarding your bricked kindle and wait for a USB HID solution.
It might be a long wait, so I my case I purchases another k4nt to replace my bricked one. I would do the same for my touch if I brick that "beyond repair by current known methods".