[solved] sysupgrade fails with "Could not get image magic"

cas

So I was trying to use sysupgrade to upgrade an O2+ to the latest firmware and kept getting "Could not get image magic" and "Image check failed." errors.

I'd copied the .bin file from my USB to my O2+ root filesystem and ran sysupgrade with the -n parameter like so:

root@Omega-C0D3:/# cp /tmp/mounts/USB-A1/omega2p-v0.2.0-b183.bin .
root@Omega-C0D3:/# sysupgrade -n omega2p-v0.2.0-b183.bin
killall: watchdog: no process killed
Sending TERM to remaining processes ... onion-helper udhcpc udhcpc dnsmasq mountd ntpd ubusd logd rpcd netifd odhcpd uhttpd device-client avahi-daemon
Sending KILL to remaining processes ...
Switching to ramdisk...
Performing system upgrade...
Could not get image magic
Image check failed.
Upgrade completed
Rebooting system...
umount: can't unmount /dev: Resource busy
[  128.911166] Removing MTD device #6 (rootfs_data) with use count 1
[  128.947452] reboot: Restarting system

After a fair amount of head scratching and frustration (including comparing md5sums of the source and the destination files), I found that running it directly from the USB drive worked:

root@Omega-C0D3:/tmp/run/mountd/sda1# sysupgrade -n omega2p-v0.2.0-b183.bin
killall: watchdog: no process killed
Sending TERM to remaining processes ... onion-helper udhcpc udhcpc dnsmasq mountd ntpd ubusd logd rpcd netifd odhcpd uhttpd device-client avahi-daemon
Sending KILL to remaining processes ...
Switching to ramdisk...
Performing system upgrade...
Unlocking firmware ...

Writing from <stdin> to firmware ...
Upgrade completed
Rebooting system...
umount: can't unmount /tmp/run/mountd/sda1: Resource busy
umount: can't unmount /tmp/run/mountd: Resource busy
umount: can't u[  484.971767] Removing MTD device #6 (rootfs_data) with use count 1
nmount /dev: Resource busy
umount: can't unmount /tmp: Resource busy
[  485.008580] reboot: Restarting system

Thought I'd put this out there in case anyone else runs into the same problem....

[edit]
Ok, so the root of the problem (pun not intended) actually appears to be that you can't run the sysupgrade command if the firmware binary is in your root (/) directory.
Observe:

BusyBox v1.26.2 () built-in shell (ash)

   ____       _             ____
  / __ \___  (_)__  ___    / __ \__ _  ___ ___ ____ _
 / /_/ / _ \/ / _ \/ _ \  / /_/ /  ' \/ -_) _ `/ _ `/
 \____/_//_/_/\___/_//_/  \____/_/_/_/\__/\_, /\_,_/
 W H A T  W I L L  Y O U  I N V E N T ? /___/
 -----------------------------------------------------
   Î©-ware: 0.1.9 b159
 -----------------------------------------------------
root@Omega-C0D3:/# cp /tmp/mounts/USB-A1/omega2p-v0.2.0-b183.bin .
root@Omega-C0D3:/# sync
root@Omega-C0D3:/# sysupgrade -n omega2p-v0.2.0-b183.bin
killall: watchdog: no process killed
Sending TERM to remaining processes ... uhttpd device-client onion-helper udhcpc avahi-daemon dnsmasq udhcpc mountd ntpd ubusd logd rpcd netifd odhcpd
Sending KILL to remaining processes ...
Switching to ramdisk...
Performing system upgrade...
Could not get image magic
Image check failed.
Upgrade completed
Rebooting system...
umount: can't unmount /dev: Resource busy
[  373.914390] Removing MTD device #6 (rootfs_data) with use count 1
[  373.951674] reboot: Restarting system

But if we move the firmware file from the root (/) directory to root's home directory (/root), hey presto, no problem writing it, as you can see here:

BusyBox v1.26.2 () built-in shell (ash)

   ____       _             ____
  / __ \___  (_)__  ___    / __ \__ _  ___ ___ ____ _
 / /_/ / _ \/ / _ \/ _ \  / /_/ /  ' \/ -_) _ `/ _ `/
 \____/_//_/_/\___/_//_/  \____/_/_/_/\__/\_, /\_,_/
 W H A T  W I L L  Y O U  I N V E N T ? /___/
 -----------------------------------------------------
   Î©-ware: 0.1.9 b159
 -----------------------------------------------------
root@Omega-C0D3:/# mv omega2p-v0.2.0-b183.bin /root/
root@Omega-C0D3:/# cd /root
root@Omega-C0D3:~# sysupgrade -n omega2p-v0.2.0-b183.bin
killall: watchdog: no process killed
Sending TERM to remaining processes ... onion-helper udhcpc udhcpc dnsmasq mountd ntpd ubusd logd rpcd netifd odhcpd uhttpd device-client avahi-daemon
Sending KILL to remaining processes ...
Switching to ramdisk...
Performing system upgrade...
Unlocking firmware ...

Writing from <stdin> to firmware ...
Upgrade completed
Rebooting system...
umount: can't unmount /dev: Resource busy
[  215.389374] Removing MTD device #6 (rootfs_data) with use count 1
[  215.425706] reboot: Restarting system

[snip]

BusyBox v1.25.1 () built-in shell (ash)

   ____       _             ____
  / __ \___  (_)__  ___    / __ \__ _  ___ ___ ____ _
 / /_/ / _ \/ / _ \/ _ \  / /_/ /  ' \/ -_) _ `/ _ `/
 \____/_//_/_/\___/_//_/  \____/_/_/_/\__/\_, /\_,_/
 W H A T  W I L L  Y O U  I N V E N T ? /___/
 -----------------------------------------------------
   Î©-ware: 0.2.0 b183
 -----------------------------------------------------
root@Omega-C0D3:/#

Hopefully less head-scratching for someone else (and I've marked the topic title [solved] for anyone finding this from google as a google search didn't have a lot of hits on this one).

[/edit]