Have just installed 6.7.1CD to USB-Flash. I got a list of error messages at the end, and don't know if they're of any significance:

cp: reading 'boot/isolinux/'loader.img': input/output error
cp: reading 'boot/isolinux/'grub.exe': input/output error
cp: reading 'boot/isolinux/'isolinux.bin': input/output error
cp: reading 'boot/isolinux/'linux': input/output error
cp: reading 'boot/isolinux/'linux64': input/output error
cp: reading 'boot/isolinux/'memdisk': input/output error
cp: reading 'boot/isolinux/'memtest': input/output error
cp: reading'KNOPPIX/KNOPPIX': input/output error

It's been some months since I tried to create a persistent image on USB-Flash, and I can't remember or find the info on how to do it. And I'm also wondering whether it's impossible because of those errors reported, the meaning of which is Greek to me.