I have just finished editing 15 short videos, now signed off by client. I want to archive everything onto an external HD and delete it all from my iMac. What's the best procedure to follow so that if in future I need to come back to the project (unlikely) I can do so easily?

Inside FCP X, go to each Project, right-click, select "Duplicate Project" (Command-D).
Specify the drive to copy it to.
Select "Duplicate Project And Referenced Events".

When done, you can then move all of those Events and Projects from their original location to the trash.

I'l just quickly point out that archiving to a Hard Drive for long term storage is a bad idea unless you remember to spin up all your drives once a year. Over time, hard drives demagnetize and unless used, all the data is lost. Lost some negative scans from a film I did a number of years ago before I knew this was the case.

Recently, I invested in a Blu-Ray burner so I can burn two or three discs and archive my projects, but the ideal solution would be LTO tape. If your clients want a backup and archive, you should consider investing and charge them for it.

Sorry been away - thanks for the replies. Regarding losing data from HDs. I've always archived projects onto 2 x HDs. I have some now that are 8 years old, and no data loss has occurred (yet!) mind you they probably do get spun up at least once a year. I might look into blu-ray, but how long is that technology going to be around? At least with an HD, even if the connectors change or become obsolete you can always take the case apart and plonk the drive into a caddy.