It happens regardless of whether you're working on files on the laptop hard drive, or on the external hard drive, doesn't it? Is the external hard drive a USB one? I've heard some things about problems with LR on Mac and "colostomy bag" drives.
I can't believe this is the issue, but maybe delete Lightroom's Preferences file - in User / Library / Preferences. Then try again.
Yes, internal or external drive makes no difference. My external drive for these images is a USB type 500gb lacie porsche.
I deleted the preferences file, same issue occurs in both catalogs, both drives.