/usr/ports/obj is where the build takes place on a default 4.6 system. A thumb drive won't be enough, unless you have a nice big one. JDK takes a number of GB to build.
You can redirect your build by setting the WRKOBJDIR environment variable. The ports(7) man page is a great place to learn about this sort of administrative control.
Hint: You can use the environment variable on a temporary basis from within your shell, or for use with make(1), you can create an /etc/mk.conf file to house them permenantly, for all users.
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