I just picked up my new Pixma TR8500 for use with my iMac, yesterday. I run high sierra on my mac. The quick access scan folder has a spot where you can specify location to save scans of different kinds. there is a little pulldown to add a folder...
This happens to me sometimes, it's not a printer problem, but in case it helps anyone.... I have two wireless networks in my house. It my signal is weak, sometimes my computer jumps to the other network (yes, even after I asked it to forget that on...