What format should an external hard drive use to be visible and working when connected to an AirPort Extreme?
The USB port on the AirPort Extreme only supports two formats......Mac OS Extended (Journaled) and FAT 32. No other formats are supported.
Remember that even the most recent AirPort Extreme was designed in 2012. APFS was still years away at that point.
Your next question might be whether Apple will update the firmware to allow APFS on the AirPort Extreme. Since the last update was about a year and a half ago and Apple has offered no updates since that time, my opinion would be that we saw the last of updates about a year and half ago.
What format should an external hard drive use to be visible and working when connected to an AirPort Extreme? My computer is running Big Sur, with the new format. APFS is not visible.
