The link I sent you above with the instructions I have only used with raspbian based SM Image and the older SM image based on Ubuntu 18.04, I have not tried with newer ones so can’t comment on those...
As for the SDA disk you need to find out the number of your external SSD drive and I put that there, if there are no other drives attached it will be sda1....it WONT be just sda there will be a number after it..
Stellarmate OS on Raspberry pi4b
Skywatcher EQ8 pro, on steel pier
Takahashi FSQ85, FS60cb & Meade 8” SCT (de-forked)
Starlight Xpress SXVR H18, SXVR M25c, Lodestar x2 Guide Camera
Pegasus Ultimate Hub V2 for all USB & Power
Pegasus focus motors on all scopes