The current RAID contains 3 disks:
root@penguin2:~# cat /proc/mdstat md0 : active raid5 sdb1[1] sda1[0] sdg1[4] /dev/md0 33T 27T 4.2T 87% /export
using /dev/sda,sdb,sdg
The current root and swap is on
# root /dev/sdd1 393G 121G 252G 33% / # swap /dev/sdd5 partition 976M 76.5M -2
We can therefore add four new disks in slots /dev/sdc,sde,sdf,sdh
penguin2 has no out-of-band and no serial connector right now. That means any work needs to be done on the terminal.
Boot loader menu:
menuentry 'Debian GNU/Linux' --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-7ff268df-cb90-4cbc-9d76-7fd6677b4964' {
        load_video
        insmod gzio
        if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
        insmod part_msdos
        insmod ext2
        set root='hd2,msdos1'
        if [ x$feature_platform_search_hint = xy ]; then
          search --no-floppy --fs-uuid --set=root --hint-bios=hd2,msdos1 --hint-efi=hd2,msdos1 --hint-baremetal=ahci2,msdos1  7ff268df-cb90-4cbc-9d76-7fd6677b4964
        else
          search --no-floppy --fs-uuid --set=root 7ff268df-cb90-4cbc-9d76-7fd6677b4964
        fi
        echo    'Loading Linux 5.10.0-18-amd64 ...'
        linux   /boot/vmlinuz-5.10.0-18-amd64 root=UUID=7ff268df-cb90-4cbc-9d76-7fd6677b4964 ro  quiet
        echo    'Loading initial ramdisk ...'
        initrd  /boot/initrd.img-5.10.0-18-amd64
}
Added to sdd MBR
root@penguin2:~# grub-install /dev/sdd Installing for i386-pc platform. Installation finished. No error reported.