If you move these folders outside of Lr, it will loose the connection to the folders and have to be reset. You can always move them inside Lr to the new location, however there have been reports of this actually losing folders completely, but this seems to be an intermittent problem!
I would move them outside of LR, then use the information in the following link to reconnect them. Either way, make sure you have an up to date backup of the photos and the folders to be safe!
When moving picture folders to the external hard drive use COPY & PASTE. This allows comparing the top-level folder properties to confirm that all of the files and subfolders were copied over successfully: Re: How do I move my pictures to external hard drive?
Many Thanks Alfred, I used the second method ie pointing lightroom to the new destination which amazingly seemed to be instant then with bated breath deleted all 41,000 photos from my hard drive. Very kind of you to help, Best Andy
Many thanks for your advice and help, i had indeed already used the copy and paste method to transfer the files before deleting. Best Andy