That message implies an I/O problem; i.e. input and output. If it's a spot-on honest message, one of the drives could have problems. Run a quick disk check on the source disk and do a full format on the target disk. This is to minimize possible problems. Hopefully the target disk does not have other files in it.
It's also better to disconnect all other storage devices and only keep the system drive and the target drive for backup. Sometimes Windows backup refuses to exclude other drives causing a huge backup package.
I recommend having an external drive exclusively for system backup if it's critical to have the system never lost. An internal drive used thru a docking or an external drive set, both work fine but for different preferences. I do that.
Windows backup makes so many files in a folder if you chose to also backup an image of the system calling that folder an image along with it. Not choosing to make an image in the process will only result in having a backup package for other files.
A repair disk is a disk to use if Windows cannot be accessed to start the utility to make the restore process. You use that repair disk to boot the computer and then select the system image thru it. It's like a minimized Windows setup application. You could even format your computer and use that image to restore it. Just make sure the image was done properly. And for your information, if you have a Windows installation disk (has to be the same Windows your using), you can use it as a repair disk. A repair disk has other tools too, not just the tool to make the restore.
The format of Windows backup is like two elements if my memory serves me right; one folder and one file. The files is an executable, but I always use the image restore utility and never tried using that executable directly.
Good luck!