Backing up Lightroom

[MarilynNS]

I have a 2015 iMac which is struggling to run the new version of Lightroom. It is also crashing when doing other things which are not so memory hungry. I have a 2Tb external LaCie hard drive onto which I would like to back up my Lightroom catalogue before the the whole thing dies on me! Presumably then all my photos will then be safely stored on the external drive and I can take the iMac to the Apple techs for an MOT. How do I export my catalogue onto the external hard drive so that Lightroom knows where they are?
Thanks Marilyn

[PhilM]

Simplest way would be to find the folder on your imac that you store your images and copy the folder (copy not cut) and then paste the folder to your external drive.

[colinw]

I would just use TimeMachine to backup everything to an external drive. My iMac does this every hour. I have had a couple of occasions when it has been a godsend.