I have a lenovo14w that has a eMMC 64GB.

I had UBUNTU on it and it was working two days ago. I'm trying install Windows 10 but the windows setup does not show the eMMC. I reinstalled UBUNTU again and when I rebooted it says "Default Boot Device Missing or Boot Failed". I'm suspecting the eMMC is dead but how can I confirm? I also can see the eMMC in UBUNTU Gparted and on the Laptop's BIOS. It's very hard to troubleshoot it as I have not access to an Operating System.

first try to boot Ubuntu live without installing it.

I guess those laptops come with extra SD-CARD slot, you need to chk it too if it has something (remove it)

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