westfw:
Um. I did some poking around, and it DOES look like the failing to upload pages of 0xFF was removed as part of the Memory Tagging Patch
Try turning on very verbose uploading (-v -v -v -v) to see if it's actually doing it, and whether it says anything informative. (I supposed it could truncate "ends of files containing only 0xFF" elsewhere in the code, but that seems wrong..)
If I remember correctly, flashing Blink from command line worked but I'll try again.
I'll definetly try the ultra verbose output and check for a newer version of avrdude I'm using the one that came with my Arduino IDE I installed a while ago. Thanks for the informative answer!
Can it be that the file I want to flash doesn't fit next to the bootloader? Is the bootloader located in the flash memory?
My file is 16050 bytes and flash has 16384 ... now I don't know what bootloader is present and how big it is.