In reply to marky:
I swapped from internal to external drives for the photos a few years ago (retained LR and the catalogue on the internal drive) and it was easy to do once you plan and set it up first. As mentioned watch out for the drive letter that you set up the external drive on. I have the drive letter marked on the drive itself as a reminder as I also backup the external drive to another external drive (which of course has a different letter) and plug them in the correct order to use. Windows still manages to change it occasionally for whatever reason, but you soon know as LR comes up with an error.
Also, personally, I back up the catalogue itself occasionally to a separate external drive just in case internal one goes down.
> thanks is it best to copy the existing files on PC to hard drive via windows or Lightroom?
Assuming nothing has changed since I did it, it can be either, iirc, though there is a slightly different steps involved. I think I read that Abode suggest using windows/Mac to transfer now a days as some used to have problems using LR. Plenty of tutorials online of how to do it. Can’t remember now which way I did it but I know it failed the first time and so I had to start again; importance of having a backup!
No matter which way you decide to use, I would suggest the safety route and retain a complete copy of your photos before you start. That way if it fails you still have retained your photos and can start again. Remember you may need to update folder location before using LR again if the catalogue looses the link depending how you proceed.
Once the transfer is done and working, it’s back to using LR for any future shifting within your overall folder to retain the catalogue links.