This just happened to me after a reboot on 6.10.0-rc1. Easy to add root=sda in the syslinux.cfg, but saw there was a post on it so just wanted to bring it up as something that's still occurring (though it seems rare).
This also happened to me updating to 6.10.2 (I reset back to 6.9) and then again when coming to 6.10.3.
Tried to upgrade from 6.5.3 to 6.6 and won't boot up after reboot
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This also happened to me updating to 6.10.2 (I reset back to 6.9) and then again when coming to 6.10.3.