ok, back up and running mostly! heres how in case anyone else has a similar problem in future.....
to "fix" i flash my boot microsd card with jun's 6.2 loader for DS3615xs - v1.03b and downloaded DSM_DS3615xs_24922.pat off the synology firmware site
booted, pressed C when the grub boot menu appeared and set vid/pid/sn (a new generated one from https://xpenogen.github.io/serial_generator/index.html )/mac1/mac2 and booted the second (recovery) option.
after a few mins, it was detectable in synology assistant (and the find.synology.com site)... from there i was able to choose REINSTALLATION from the select install type menu. i then pointed it manually at the 24922 pat file. it formatted system partitions, then copied the contents of the PAT file. after around 10 mins i wasnt able to connect.
at this point i discovered the server wouldnt properly shut down/reboot, so i had to power it off/on manually. it used to work ok with the original install, any idea whats changed, or what i need to do to sort that out?
after the reboot, the server appeared again in synology assistant, and i was able to connect and set it back up from scratch. i'm now going through and trying to remember how i had things configured (i only backed up a couple of apps configs. doh!)
so yeah. i'm back up. thanks! but if anyone knows why it isnt shutting down/rebooting, that would be useful, i dont want to have to go into the basement comms cab each time i need to reboot it
its a hp microserver gen8 and it worked ok on 5.2, and i've not changed any bios settings or anything. when SSHing onto it, even a sudo shutdown -h now just kills the ssh connection, and i have to manually power it off.
