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/dev/sda4 does not exist
First thing to try - Go into bios and change the boot order of the hard disks. That might work.
Maybe a better solution is to unplug the new disk, reboot and edit /etc/fstab to use UUID instead of device names.
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/boot/grub/grub.cfg
linux /vmlinuz root=/dev/sda4
Change to:
linux /vmlinuz root=UUID=*
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Check /etc/default/grub to make sure that UUID is not disabled for update-grub. The line should be commented like this:
# Uncomment if you don't want GRUB to pass "root=UUID=xxx" parameter to Linux
#GRUB_DISABLE_LINUX_UUID=true
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