Silly quick question, once you’ve started a job off the SD card can you close Pronterface/Printrun and shut down your computer? My Melzi doesn’t need the USB for power as it uses the printers PSU (its a Nophead Mendal90). I have a feeling I can do it but I don’t want to ruin 3 hours worth of print job 
Yep, you’re good. I’d still wait until this job completes, and do it at the beginning of a new job to make sure. Some controllers may say they’ll run from the PSU but occasionally they need a solder jumper to turn on that functionality and removing USB power could interrupt it.
Yeah I took the plunge and closed Printrun but I think I’ll leave the PC on just in case this time. I have the jumper set to not require USB power but like you say you never know.
I do most of my printing without a computer even connected. You don’t need an LCD to do this with marlin, just name the gcode file you want to print “auto0.g” on the SD card, and it will start automatically on power-up/reset.
@Whosa_whatsis hah, didn’t know about that little trick! Nice!
I didn’t know about it either until I started trying to implement it myself, and when I found the proper place to insert the code, I discovered that it was already there.
That’s awesome! Thanks @Whosa_whatsis now I can move the printer off my desk and not have USB cables trailing