You can let the Lightroom 5 catalog be at its original place and you can import the images from Aperture/Iphoto however, at the time of import you can specify the copy location as your external drive so that the original images are copied on your external drive.
Refer to this article for the instructions : Aperture Import plug-in for Lightroom FAQ
thanks for your prompt reply!
will the LR imports be copies?
will there still be images in Aperture?
I appreciate your help...I find all this overwhelming...and I worry about preserving my photos!
As you will import through Lightroom, those images will be copied over to the location selected at the time of import.
Yes, the images will still remain in Aperture.
You will not loose them from aperture.
thank you thank you thank you!
Glad to help