the problem seems to be that the disk isnt being mounted in /mnt/usb upon reboot.
when it is disconnected and re-connected it appears as
/dev/sdb1 on /mnt/usb/usb0 type ext2 (rw,sync)
/dev/sdb2 on /mnt/usb/usb1 type ext2 (rw,sync)
/dev/sdb3 on /mnt/usb/usb2 type ext2 (rw,sync)
/dev/sdb4 on /mnt/usb/usb3 type ext2 (rw,sync)
i think the partitions are fine as per this readout from disk utility tool.
here is also serial bootlog
thanks
hputty.logdisk config.txt