BTT_SKR_13_14_14T_SD-DFU-Bootloader VS Marlin-Ender-5-Plus

Take a look at https://github.com/GadgetAngel/BTT_SKR_13_14_14T_SD-DFU-Bootloader -- it may help. It's not only for the larger boards.

However, I couldn't manage to find the original bootloader for this chip. I found this : https://github.com/GadgetAngel/BTT_SKR_13_14_14T_SD-DFU-Bootloader which is very interesting for many older BTT controllers, but not the SKR 3 EZ.

If you want to investigate, you might start by looking at https://github.com/GadgetAngel/BTT_SKR_13_14_14T_SD-DFU-Bootloader

The only other way is to use a little programmer such as an ST-Link, which looks similar to but is different from the little USBasp programmers that can be used for 8-bit boards. An ST-Link can flash firmware directly, but it's not as simple as using the SD card. See, for example, https://www.youtube.com/watch?v=7O_ZOdnM0SU and https://github.com/GadgetAngel/BTT_SKR_13_14_14T_SD-DFU-Bootloader

For LPC based boards this may help. It seems to allow flash over USB.

It doesn't sound like an EEPROM fault, because that only affects various settings, not what functions and codes it supports. It sounds like something is wrong with the bootloader, eg it's reading the firmware file but not storing it. If nothing else works, is it worth getting a little STLink in-system programmer and reinstalling the bootloader and firmware completely? There's a link at https://github.com/GadgetAngel/BTT_SKR_13_14_14T_SD-DFU-Bootloader which should be helpful if you want to try that.

(If all else fails, you can try reflashing the bootloader - something on this page may help you.)

Technically, it's possible to use a programmer to update the firmware, and I've seen descriptions of how to do that with various small programmers. Here's an example. Perhaps a google search will find more information, given that it does exist. People have used that technique to read and (re)load bootloaders and firmware.

I would suggest starting with this: https://github.com/kennethrrosen/Marlin-Ender-5-Plus