I have a Debian system on /dev/sda1 (/) and /dev/sda2 (extended, /dev/sda5 is /home, /dev/sda6 is swap). I want to install Gentoo in parallel, so I shrinked sda2 and created /dev/sda3, in which I installed Gentoo, and /dev/sda4, to serve as Gentoo's /home:Disk /dev/sda: 931.5 GiB, 1000204885504 bytes, 1953525167 sectorsUnits: sectors of 1 * 512 = 512 bytesSector size (logical/physical): 512 bytes / 512 bytesI/O size (minimum/optimal): 512 bytes / 512 bytesDisklabel type: dosDisk identifier: 0xeb362e8fDevice     Boot      Start        End    Sectors   Size Id Type/dev/sda1  *          2048   29296639   29294592    14G 83 Linux/dev/sda2         29298686 1673951231 1644652546 784.2G  5 Extended/dev/sda3       1673951232 1735391231   61440000  29.3G 83 Linux/dev/sda4       1735391232 1953523711  218132480   104G 83 Linux/dev/sda5         29298688   35547135    6248448     3G 82 Linux swap / Solaris/dev/sda6         35549184 1673949183 1638400000 781.3G 83 LinuxPartition table entries are not in disk order.I skipped the bootloader section of Gentoo's Handbook as I wanted to use my current grub2 install on /dev/sda1, so after finishing the installation I booted to Debian and ran update-grub, which successfully added Gentoo to the menu. However, when booting to it I get a kernel panic and a message about it not being able to open root device:fstab for Debian and fstab for Gentoo. I tried editing Gentoo's fstab to use UUIDs like Debian, to no avail. I also tried editing boot options for Gentoo in grub. Original:### BEGIN /etc/grub.d/30_os-prober ###menuentry 'Gentoo Base System release 2.2 (on /dev/sda3)' --class gnu-linux --class gnu --class os $menuentry_id_option 'osprober-gnulinux-simple-1c46e4aa-e486-48ba-8f61-1484ed899e1e' {        insmod part_msdos        insmod ext2        set root='hd0,msdos3'        if [ x$feature_platform_search_hint = xy ]; then          search --no-floppy --fs-uuid --set=root --hint-ieee1275='ieee1275//sas/disk@0,msdos3' --hint-bios=hd0,msdos3 --hint-efi=hd0,msdos3 --hint-baremetal=ahci0,msdos3  1c46e4aa-e486-48ba-8f61-1484ed899e1e        else          search --no-floppy --fs-uuid --set=root 1c46e4aa-e486-48ba-8f61-1484ed899e1e        fi        linux /boot/vmlinuz-4.1.12-gentoo root=/dev/sda3}submenu 'Advanced options for Gentoo Base System release 2.2 (on /dev/sda3)' $menuentry_id_option 'osprober-gnulinux-advanced-1c46e4aa-e486-48ba-8f61-1484ed899e1e' {        menuentry 'Gentoo Base System release 2.2 (on /dev/sda3)' --class gnu-linux --class gnu --class os $menuentry_id_option 'osprober-gnulinux-/boot/vmlinuz-4.1.12-gentoo--1c46e4aa-e486-48ba-8f61-1484ed899e1e' {                insmod part_msdos                insmod ext2                set root='hd0,msdos3'                if [ x$feature_platform_search_hint = xy ]; then                  search --no-floppy --fs-uuid --set=root --hint-ieee1275='ieee1275//sas/disk@0,msdos3' --hint-bios=hd0,msdos3 --hint-efi=hd0,msdos3 --hint-baremetal=ahci0,msdos3  1c46e4aa-e486-48ba-8f61-1484ed899e1e                else                  search --no-floppy --fs-uuid --set=root 1c46e4aa-e486-48ba-8f61-1484ed899e1e                fi                linux /boot/vmlinuz-4.1.12-gentoo root=/dev/sda3        }}### END /etc/grub.d/30_os-prober ###I tried using set root=/dev/sda3 and set root=sda3 but honestly I have no idea what that means, it just seemed related to the panic message (I'm a complete noob when it comes to bootloaders and the sort). So what's going on here and how should I proceed? It's worth mentioning I'm booting from an external hard drive (/dev/sda) plugged in via USB (my internal one went kaput). Gentoo install went without any errors/warnings, and I don't think I forgot to compile anything into the kernel but I guess it's possible... ext4, SCSI and USB support I'm pretty sure was included (not as module).
Kernel panic after Gentoo install -- cannot open root device
kernel;boot;partition;grub2;gentoo