65536 bytes (66 kB) copied, 0.000247394 s, 265 MB/s
INFO    : Kernel partition image emitted: /mnt/host/source/src/build/images/buddy/R52-8218.0.0/hd_vmlinuz.image
INFO    : Root filesystem hash emitted: /mnt/host/source/src/build/images/buddy/R52-8218.0.0/rootfs.hash
INFO    : Kernel image A   size is 4908032 bytes.
INFO    : Kernel image B   size is 4908032 bytes.
INFO    : Kernel partition A size is 16777216 bytes.
INFO    : Kernel partition B size is 16777216 bytes.
INFO    : Appending rootfs.hash (10113024 bytes) to the root fs
INFO    : Extracting the kernel command line from /mnt/host/source/src/build/images/buddy/R52-8218.0.0/vmlinuz.image
syslinux: short read
ERROR   : Wed Apr 20 06:23:16 PDT 2016
ERROR   :  PGID  PPID   PID     ELAPSED     TIME %CPU COMMAND
ERROR   : Arguments of 2246:  '--arch=amd64' '--to=/mnt/host/source/src/build/images/buddy/R52-8218.0.0/chromiumos_base_image.bin' '--from=/mnt/host/source/src/build/images/buddy/R52-8218.0.0/rootfs_dir/boot' '--vmlinuz=/mnt/host/source/src/build/images/buddy/R52-8218.0.0/rootfs_dir/boot/vmlinuz' '--to_offset=127926272' '--to_size=16777216' '--kernel_partition='/mnt/host/source/src/build/images/buddy/R52-8218.0.0/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.0qW6pl: not mounted
WARNING : Initial unmount failed. Possibly crosbug.com/23443. Retrying
umount: /tmp/esp.0qW6pl: not mounted
INFO    : Unmounting image from /mnt/host/source/src/build/images/buddy/R52-8218.0.0/stateful_dir and /mnt/host/source/src/build/images/buddy/R52-8218.0.0/rootfs_dir
Cleaning up /usr/local symlinks for /mnt/host/source/src/build/images/buddy/R52-8218.0.0/stateful_dir/dev_image
ERROR   : Wed Apr 20 06:23:23 PDT 2016
ERROR   :  PGID  PPID   PID     ELAPSED     TIME %CPU COMMAND
ERROR   : Arguments of 530:  '/mnt/host/source/src/build/images/buddy/R52-8218.0.0' 'chromiumos_base_image.bin' '--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/buddy/R52-8218.0.0/chromiumos_base_image.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.
06:23:24: ERROR: 
return code: 1; command: /b/cbuild/internal_master/chromite/bin/cros_sdk 'PARALLEL_EMERGE_STATUS_FILE=/tmp/tmpCGRIhg' 'USE=-cros-debug chrome_internal afdo_use' 'FEATURES=separatedebug' 'CHROME_ORIGIN=LOCAL_SOURCE' -- ./build_image '--board=buddy' --replace '--version=8218.0.0' base test
cwd=/b/cbuild/internal_master, extra env={'PARALLEL_EMERGE_STATUS_FILE': '/tmp/tmpCGRIhg', 'USE': '-cros-debug chrome_internal afdo_use', 'FEATURES': 'separatedebug', 'CHROME_ORIGIN': 'LOCAL_SOURCE'}

Phobbs, this looks like an infra issue to me. Can you check if it is?