Updating Windows Defender Offline Tool

A previous thread mentioned that the offline tool wizard could incorporate new definitions onto the USB boot disk. However, when I created the disk the tool warned me that everything on the disk would be wiped off. How will the updating process avoid that warning?

In addition, I have a WinXp/32 and a Win8/64 computer. Do I need to create a separate USB boot stick for each computer?
 
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