EDIT: problem solved – flashing just wouldn’t work on the laptop, but when I tried on the desktop it ran no problemo on the first go! I’ll leave this here if it helps anybody. Moral of the story is, never give up, try everything and keep the faith
Quick intro: bought used Xperia 10, found it has Android 10, managed a downgrade after 6+ hours of tries, dev options / usb debugging enabled and persists after reboots, bootloader unlocked successfully. On Linux, also tried on Windows.
So after all this, I’m ready to flash SFOS X. Hit a brick wall when fastboot just gives ‘Failed to get boot partition size’. Tried adding --force inside Jolla’s script, then tried manually with fastboot --force flash boot_a imagename. Tried to fastboot getvar:anything manually, but the phone won’t give out anything. It does give ‘already unlocked’ if i do fastboot oem unlock again, so it’s connected. Tried all this on my cozy Linux (fastboot from Arch AUR) and Windows with Jolla’s provided fastboot and a bunch of stupid USB drivers from xperiastockrom.com and Google. Set up an udev rule on Linux, so I’ve been able to try everything both as regular user and superuser.
I’m at my wit’s end, please somebody help. The image I flashed to bring the phone back to Android 9 was a ‘Commercial / Journalists’ image from XperiFirm. Is it likely this is the one image that has fastboot / usb debugging fudged somehow?