how to use macrium reflect to back up to a network file

It's the one that has an earlier backup on it. I'm saving it to a separate usb-connected backup dive with 764 Gb of free space on it.

If you don't use the scheduling function (or it's not available in your version) to run background back-ups at certain times of day or you only ever run the same back-up operation manually then you may well get away without using the definition files. So I have 6 definitions for different tasks. Plus I have a USB drive I use occasionally for both file/folder back-ups and images which need a further couple of definition files. I also run a monthly full system image to both drives which requires two more definition files. I run daily scheduled back-ups to both drives separately which requires two definition files. I have the paid for version of Reflect and two NAS drives.

Saving back-up definitions as XML files is handy if you run multiple back-ups. You could try creating an entirely new back-up from scratch and seeing if that runs. A change like that could easily go unspotted if you open up Windows File Explorer to look at the drive contents. In particular if the definition was made when the USB drive was assigned drive letter E: say it will fail if Windows decides to assign drive letter F: instead. Go through the configuration of the back-up and check all file locations that are specified in the config do in fact exist.

How to use macrium reflect to back up to a network file