Comment 7 for bug 557388

And observing https://bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/877032 the user says the filesystem.size file is indeed there in the casper folder on the filesystem in spite of getting this error, so yes, this indeed seems to be users deciding to unmount the filesystem rather than UNetbootin leaving the file out. Perhaps whatever error message Ubiquity is printing which is leading the users to decide they need to unmount the filesystem could be made more clear?