On flashing the latest image of HassioOS on Mac with etcher, on completion I see the following and cannot access the disk - any advice? When I insert the SD card into the pi HassOS does boot up, but since I cannot edit the network file via my Mac I cannot setup wifi.

You need a usb stick and not edit the SDcard :) Maybe the Mac OSX don't support GPT legacy mode.
OK the docs need updating then as states:
Optional - Setup the WiFi or static IP: On the SD-card, create the network/my-network file and follow the HassOS howto.
Where you see that? I can't find this...
It should just say
Optional - To setup WiFi or a static IP, follow the HassOS howto.
The reference to SD is incorrect.
PR has been merged. This issue may be closed now.
There are other references to the SD-card as well. The one I stumbled upon is
https://www.home-assistant.io/hassio/zwave/
Sorry if this is obvious, but why isn't it possible to open the SD-card in a mac?
The file system used by HASSOS on the SD card is not compatible with MacOS. A Linux VM on your Mac would be able to read it as a work around.
This is still a problem/confusing. Had to google it to get to this page.
The installation page still says:
"Optional - set up the Wi-Fi or a static IP address. There are two possible places for that:"
Linux VM is indeed a valid workaround.
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The file system used by HASSOS on the SD card is not compatible with MacOS. A Linux VM on your Mac would be able to read it as a work around.