For past 6 mths did you update your FW which could have caused the prob or was it new MKV files you got which you never played on the POHD before until now?

Can send me one sample to try ?

i dunno what a bootloader is... tot flashing is all that is required?

could be this issue ur facing:
if you are changing from old to new gui, it takes a few minutes (depending on no. and size of files on yr hdd) when the new gui comes on for the first time.
this is because the default setting is "autoscan on", so it takes a while to scan the hdd before it gets to normal working condition.
wait till can navigate, you can then go to setup and set autoscan to off if you so wish.
i think autoscan off is better if you are plugging and unplugging hdds often.

tried to install the 1st gui, then move up slowly?

each firmware has a pdf file that defines the lowest last version required for that version to work.

i have flashed my 2 units more than a dozen times each , so far, ok.

for old models, best to stick to 3966, later versions give strange problems.

newer ones incl pohd2 and mini2, 7317 will be fine. 7317 is a good firmware.

yes, u can. then work upwards again one by one. i have done it several times. time consuming though.
only thing is i do not know if this will solve the "bootloader" problem u suspect.