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(1) it wouldn't let me save more than a 300MB .ISO with the trial version
Sorry, this is a trial version limitation.
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when I tried to burn a modified .CIF file (from Roxio, with a file added) UltraISO told me the image file was corrupt and refused to burn the CD.
UltraISO cannot burn .CIF file directly. To resolve this problem, it is recommanded to choose 'Options'->'Configuration'->Integration'->'Detect' and set Roxio as the default burning software.