I have the big ol Seiki plotter purchased about two years ago. I can't remember the model, ST850 I think, but here have been my issues:
Distance between pinch roller bottom roller are different as you go down the length of the 40in. Pinch rollers can be adjusted slightly, but I also measured the distance from the blade housing to the bottom roller were different from one side to the other. This means I can't feed my orical at one end because the rollers don't close together. Instead I have to load it on the far end.
Drivers for the usb interface are a bit touchy. Often times the pc sees the plotter but when you send a file to cut an error message occurs saying, cannot communicate with port. Essentially the drivers set up a communication line with a specified port, basically making a usb port behave like an old style printer port. so if you plugged your plotter into a different usb port, it wouldn't work unless you reinstalled it specifying the new COM# port. So thats what I do when I get a communication error; uninstall plotter and reinstall using a different COM#
If I get to my pc, ill attach the support forum conversation I had with a Seiki tech, it is worth a good laugh.