You may have misunderstood that last post but gotten it right, anyhow, Rita.
Your best answer
is to "
leave it alone."
Since Time Machine waits patiently in the background when it can't find it's target drive you can leave your TM settings as is
and leave your external behind when you travel with your MacBook.
When you get back to your external and plug it back in, TM will immediately (I think) pick up and continue its hourly labors until the next time you yank the plug.
I hope this puts this particular issue to rest.
