usb dvd's not reformatting

I love the make a dvd a usb feature in Win7. However I've noticed on some discs (no particular brand) that I am unable to recover the disc. If I format it does but in actuality the progams, altough no longer there as far as explorer is concerned, I am left with only the space that was not used in the initial burn. Not a free usb drag and drop media . Anyone else noticed this? Is it the build I have (7100 RC) or does this feature simply not live up to it's amazing potential?
TIA
J

This link gives a detailed tutorial (With screen shots) on how to do it.

How to Install Windows 7 From USB Drive without Windows 7 ISO DVD

Also see this very nice post done by Brink on our forms.

USB Windows 7 Installation Key Drive - Create

[QUOTE=Aheaton;172196]Also see this very nice post done by Brink on our forms.

Good stuff. This is not what I am talking about however. My issue is when installing a new dvd in win 7 it asks one if they want a usb medium disc or a traditional burn explorer disc. I love this option as it makes purchasing re writables irrelevant. But as I stated sometimes it will not remove the data you have put on the disc. You can format it and Windows will think there is nothing on the dvd but it will still count the storage imprint for the initial data you've burned. Making the process useless.
So I am wondering why it works some times and not others. As in I can re format a dvd r in usb mode & reuse it as tho it were a rw. It works about 75% on my 64 machines. Aroound 30% on my x86 ones.