Check the solder connections between the SIO connector and bottom of the PCB. These are almost guaranteed to break on every XF551.
Dug out my XF551 today, it has died in storage over the last 3 months:sad:
Symptoms are: brief flash from the drive LED when turned on, Computer will not recognize that there is a drive connected.
I have used my 1050 with the same computer/discs/sio cable/psu this afternoon and all is fine with that set up, so those possibles can be ruled out.
Any ideas where a novice should start to look for the fault? (I have basic tools, multimeter, soldering iron, hammer etc)