tkaiser replied at Sat May 2, 2015 04:52
Why don't you give it a try? Since AXP209 is providing LiPo battery handling and a step-up-convert ...
But I can find no evidence that there's a step-up or boost converter, the only things I saw were buck (step-down) and LDO (low drop-out) converters. This would mean that the output voltage can never exceed the battery voltage of 3.7~4.25V, which cannot run a HDD.
Can you confirm that the USB ports are in fact supplying 5 V (instead of 3.3~4 V) when running on battery power?
If not, can you please also confirm that the (USB powered?) WiFi remains operational (and stable) when on battery power?
I guess we'd be better of using an external USB back-up battery or accept that the HDD will go offline in a power cut.
Add post (Sun May 24, 2015 12:14):
Looking at the BPi-M1 schematics, I see there's an external 2A boost converter (SY7152) for powering the USB/HDMI ports. The S-ATA port is powered directly from ACIN bypassing the AXP completely.