Failed to load font 0 on 4.2.1 - Root cause found!

I have placed the font0.bin-font4.bin files (approx 13k in size each) in the root of the SD card for my Matek F405 Wing FC running AP 4.2.1 and still when I select any different font for OSD_FONT param, it displays “OSD: Failed to load font 0” on the OSD screen, and the font remains as the default. Any tips on how to get this to work please? I never had an issue in the past with Omnibus F4 Pro v3 boards, but strangely on this Matek board I am seeing this issue with the 4.2.1. release. Unfortunately I only just fitted this board to this plane and applied the firmware update immediately, so can’t compare with previous releases.

Does no one else have this issue? I have tested now on several 1MB flight controllers and none are able to load the font files from SD card. (tested on Matek F405 Wing, Omnibus F4 Pro v2.1, Omnibus F4 Pro v3). I have also tried locating font files in both the root of SD card and also in the APM folder - no joy! Status message reports:
Failed to load font n (depending on which font you select inside Mission Planner for onboard OSD)
Failed to load font 0

I have found the root cause of the issue - MavFTP!
If you connect the model over USB cable and connect Mission Planner, MavFTP seems to lock access to the SD card in such a way that AP cannot access the font files on the SD card. Changing the OSD font in mission planner results in ‘Failed to load font’ messages I have mentioned above. If you instead connect over UDP (as I am able to do using TBS CRSF), or some other method that does not yet have the ability to use MavFTP, then AP is able to access the font files without issue. I think the devs possibly need to take note of this limitation.

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Useful information.thx

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