Differential Zip Backup Ending Too Soon

I'm performing my second differential backup (the basefile and first differential went fine) but it exits too soon after creating an 88-89Mb Zip file (which is only a few percent into the job).

I thought that perhaps there was a file that Toucan didn't like and deleted the files where the program exited but on retry the backup just ended on a file soon after. I then deleted a bunch of folders after and before the point where it exited yet the program did get further again but still exited early on the 88-89Mb mark.

I've checked for space on the external drive I'm backing up to and it's still got 96 or so gigabytes and it's NTFS. The program doesn't crash out it just says it's finished but clearly isn't. Also the resulting zip won't open as it's 'incomplete'.

Is there a way to solve this? Or is there at least a way to get Toucan/7-zip to show me more information about why it's exiting?

Thanks a million in advance,
John.