I eventually got one to boot, but I suspect I'm missing something basic.

mkinitrd couldn't find sd_mod:

# mkinitrd -o initrd.img-2.6.18-grml -r /dev/sdb3 2.6.18-grml /usr/sbin/mkinitrd: add_modules_dep_2_5: modprobe failed FATAL: Module sd_mod not found.

yaird worked for me going from 2.6.16 to 2.6.17, but now / isn't in fstab

# yaird --output=initrd.img-2.6.18-grml 2.6.18-grml yaird error: mount point not in fstab: . (fatal)

So I modified /etc/yaird/Default.cfg

< MOUNTDIR "/" "/mnt" ---

          MOUNTDIR      "/mnt/sdb3" "/mnt"

But yaird now didn't like /dev/usb-sdb3 in fstab

# yaird --output=initrd.img-2.6.18-grml 2.6.18-grml yaird error: duplicate device name in fstab: /dev/sdb3 (fatal)

So I commented it out

# Added by GRML #/dev/usb-sdb3 /mnt/usb-sdb3 auto user,noauto,nodev,noatime,nosuid,noexec,rw,uid=1000,gid=1000 0 0

And success

# yaird --output=initrd.img-2.6.18-grml 2.6.18-grml