Copy failed, both partitions deleted. Worst-case scenario.
Before anyone tells me I should have backed the data up, I was using gparted booted from the live USB to copy the laptop's main Windows partition over to a second USB stick. This was the most forensically sound method of doing a backup I could think of for this very important data. Nevertheless, after seemingly copying all of the NTFS partition from the laptop hard drive to USB, there was an error. I saved the log to the desktop and opened it in the browser to see what the error was, and I think it said "insufficient memory", although because I was booting from the live USB I have since rebooted (doh!) and have lost that log. The copy had basically finished however, with significant excess on the USB. The partition was originally 200Gb, but had immediately before copying (and a reboot) had been shrunk to just 23Gb, the minimum size possible size, so it might be something to do with that not having any buffer. Before shrinking, I had run chkdsk C: /F /X from a windows 8.1 recovery CD to sanitise the partition, because prior to that gparted had identified the partition as incorrect/damaged, which kicked this whole thing off :)
In this instance, the valuable data was Grandma's holiday photos - a loss to be sure - but if this bug is reproducible it really needs to get fixed, because with the partition information deleted for both the hard drive and the USB drive about this partition, it's basically completely gone. I did a scan for partitions in the windows recovery CD afterwards, and it found none, so i had nothing to run chkdsk on to get that back. Because gran needs her laptop for urgent.. whatever.. i've already wiped it and installed elementary OS. I still have that USB stick untouched since the failed copy however. Maybe that can be used to figure out what went wrong? Maybe this description of events is enough? Happy to help :)