All my precious data is stored on this 2TB HDD by Buffalo. When I first started computing on a Windows machine many moons ago, I experienced lots of data loss due to viruses or hardware failure. I learned that having precious data on the same drive as the system was too risky so ever since, my data has always been on a separate drive - even after switching to Mac.
With my data on this external HDD, when I travel abroad (which is once a month), I just unplug and plug into my MacBook Air. The data is regularly backed up to another external drive and also on cloud storage at Amazon S3. It costs about 60 USD per month to store 1TB of data.
60 USD may sound a lot but just imagine you lost all your family photos and videos - or even worse - your hentai_archive.zip. You would probably pay anything to get it all back right?
Its important to have redundant copies of your data on and off site. If anything happens to your house, you will have peace of mind that your data is safe and encrypted on the cloud.
What sort of data backup workflows do you have that you can share with us?
Anyway, the HDD is stored on the back of the iMac by a cheap alternative to the iMac BackPack - its just a deck box (just like the Mirai Carry Cases) taped to the back.