Is there a way to copy an entire file to an external flash drive for storage?

Syncplicity will not sync data directly to any storage that is not local and permanently attached to your system. So you can't sync to an external flash drive by selecting that drive in Syncplicity.

You can, of course, always use the file system to copy data from a Syncplicity folder on your attached storage and deposit it on the external flash drive.... you just can't sync it there in Syncplicity.