From the property's documentation ("<a href="http://doc.xceedsoft.com/products/Xceedzip/SpanMultipleDisks_property.html">SpanMultipleDisks</a> property"):
"When SpanMultipleDisks is set to xdsAlways or xdsRemovableDrivesOnly and that a removable drive is being written to, you can only use the Zip method to create a new spanned zip file. Once the spanned zip file has been created and contains files, no more files can be added to, or replaced in it."
As a workaround, set the SpanMultipleDisks property to xcdDiskSpanning.xdsNever when you are zipping on a USB Drive.
Charles Bérubé-Rémillard
Technical Support
Xceed Software Inc.