Thanks for all your help. Got it working tonight using the following steps for anyone else trying to piece it all together.
- Installed Raspbian Lite (2017-04-10) on a MicroSD card and followed the instructions here to enable USB boot mode.
- Rebooted Pi then shut down a few minutes later.
- Burned hassio Pi3 image to a 16GB USB drive.
- Booted PC into Gparted to re-partition hassio image.
- Increased /dev/sdb4 from 1GB to around 4GB (must do first otherwise you can’t expand the resin-data partition).
- Increased /dev/sdb6 from 1GB to pretty much whatever was left.
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With the modified thumb drive back in my PC, I added the following located in the resin-boot drive and also in the resin-rootA partition inside the resin-boot folder:
I changed the “root” option in cmdline.txt to:
root=/dev/sda2
and I added the usb boot line below to config.txt, right at the end:
program_usb_boot_mode=1
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I then put the reworked thumb drive back in my Pi3 without the MicroSD card and after about 1 minute, the Home Assistant logo appeared so I knew we were underway. 20 mins later, working Hassio system with no MicroSD card. Yay.