> Do you mean to redownload some of the projects?
No - the 'Continue an interrupted mirror' should not 
redownload too much data.

> Is there another safe way to backup the projects as they 
> are,with for example another burning program which 
doesn't 
> need to save the files as ISO9660?
If the files are still on hard disk, you can mirror these 
files using httrack, with ISO9660 option turned on.

Just use the <file://C:\My> Web Sites\.. syntax as starting 
URL.