I have an iMac that could theoretically hold everything and then some, but I keep all my scrap and all photos prior to 2012 on an EHD. I am actually more a photographer than a scrapper, so I have close to 30,000 photos, counting raw, jpeg, family, client, etc. Which, for a photographer, is still a low number. But all my scrap supplies and LOs live on my EHD, as well as all photos prior to 2012. I typically move prior year photos to the EHD round about June/July, but I haven't done that yet. I got this iMac at Christmas and I have plenty of HD space on it, so I haven't felt the need to move it.
My only complaint is that my EHD is not particularly fast, so there's often a lag as it dozes, and then when I want to access a file I have to wait a few seconds, but if I am actively scrapping or working from that drive it's fairly snappy.
I have a second EHD that I run Time Machine on for backup.