https://uberchromegw.corp.google.com/i/chromeos_release/builders/slippy-release-group%20release-R51-8172.B/builds/40

INFO    : Extracting the kernel command line from /mnt/host/source/src/build/images/wolf/R51-8172.47.0-a3/vmlinuz.image
syslinux: short read
ERROR   : Thu Jun  2 00:57:47 PDT 2016
ERROR   :  PGID  PPID   PID     ELAPSED     TIME %CPU COMMAND
ERROR   : Arguments of 8802:  '--arch=amd64' '--to=/mnt/host/source/src/build/images/wolf/R51-8172.47.0-a3/factory_install_shim.bin' '--from=/mnt/host/source/src/build/images/wolf/R51-8172.47.0-a3/rootfs_dir/boot' '--vmlinuz=/mnt/host/source/src/build/images/wolf/R51-8172.47.0-a3/rootfs_dir/boot/vmlinuz' '--to_offset=127926272' '--to_size=33554432' '--kernel_partition='/mnt/host/source/src/build/images/wolf/R51-8172.47.0-a3/vmlinuz.image'' '--install_syslinux'
ERROR   : Backtrace:  (most recent call is last)
ERROR   :  update_bootloaders.sh:195:main(), called: die_err_trap  
ERROR   : 
ERROR   : Command failed:
ERROR   :   Command 'sudo syslinux -d /syslinux "${ESP_DEV}"' exited with nonzero code: 1
umount: /tmp/esp.DjgRHH: not mounted
WARNING : Initial unmount failed. Possibly crosbug.com/23443. Retrying
umount: /tmp/esp.DjgRHH: not mounted
losetup: /dev/loop5: detach failed: No such device or address
INFO    : Unmounting image from /mnt/host/source/src/build/images/wolf/R51-8172.47.0-a3/stateful_dir and /mnt/host/source/src/build/images/wolf/R51-8172.47.0-a3/rootfs_dir
ERROR   : Thu Jun  2 00:57:58 PDT 2016
ERROR   :  PGID  PPID   PID     ELAPSED     TIME %CPU COMMAND
ERROR   : Arguments of 7097:  '/mnt/host/source/src/build/images/wolf/R51-8172.47.0-a3' 'factory_install_shim.bin' '--use_dev_keys' '--adjust_part='
ERROR   : Backtrace:  (most recent call is last)
ERROR   :  cros_make_image_bootable:427:main(), called: make_image_bootable '/mnt/host/source/src/build/images/wolf/R51-8172.47.0-a3/factory_install_shim.bin' 
ERROR   :  cros_make_image_bootable:1:make_image_bootable(), called: die 'cros_make_image_bootable failed.' 
ERROR   : 
ERROR   : Error was:
ERROR   :   cros_make_image_bootable failed.