backup failed

Have been using 32gb ixpand fine for two months. Deleted some files off it (a lot) then tried to back up my iPhone. It was a big backup and took several attempts as it kept getting stuck. Now the backup is seeming to go through with the final 350 files, then as it finishes a quick message pops up (blink and you miss it) to say ‘backup failed’. It’s also started warning me that my battery is low and might run out before the backup is complete, which it’s never done before (I’m sure I’ve backed it up with lower battery before!).

In process of updating IOS to 9.3 to see if that fixes it, any other ideas? the iXpand is not full - it had 9gb free before I deleted around 13gb of stuff off it

ios update hasn’t helped

also when I try to export my favourites folder to the ixpand drive every single file says ‘FAILED’ in red