Windows XP SP3 with SSE2, SSE3 (yeah, shopping for new laptop)
On both Thunderbird and Seamonkey after using enigmail and closing cannot do 'Safely Remove Hardware'. Using Task Manager find that gpg-agent.exe is still running. End process and now can do 'Safely Remove Hardware'. Note: Before upgrade to new Thunderbird, Seamonkey and enigmail would also get multiple istances of gpg2.exe leftover. Thanks.
[ UPDATE] Have confirmed this also happens on Windows 8.1. gpg-agent.exe remains active even after enigmail and Thunderbird or Seamonkey are closed. Cannot eject thumbdrive till end process.