This seems to be a common issue and has been going for some time. Using the ubuntu-11.04-desktop-i386.iso I am still having issues with this.
The way I have mounted the casper-rw file is doing the following.
- Mount your USB key
- sudo fsck -y -t ext3 /media/[drive]/casper-rw
Some users, including myself, are still reporting issues even after doing this so ymmv.
You might want to start backing up your loop file and keeping it only as large as you need.
Before doing this resize make sure you back it up but here are the basics.
- Boot without using persistent or use another system
- Insert and mount your device that contains the loop file
dd if=/dev/zero bs=1M count=1024 >> casper-rw
- Where 1024 is larger than your current used
Using this method you should be able to keep your loop files only as large as needed, have some backups and if all else fails be able to fsck them.