You will need to repoint the folders in the Folder's panel to the EHD.
This works best if you photos are in folders that all nest under a single or just a few Parent folders. Then you can simply repoint the top level folder and all the subfolders will follow. If not, you will either have to expose a hidden Parent folder (if one exists) or repoint each folder in the Folders panel individually.
If you can see 2012 but not Photographs in this example you can Right/CTRL Click on the Folder and tell Lightroom to Show Parent Folder.
Go to the Folders panel
Select a Folder
Right Click (PC) or CTRL Click (Mac) on the folder icon.
Choose Update Folder Location…
Navigate to the folder's location on the EHD
Press [Select Folder]
Repeat as necesary.
Test one or two folders. You can always point them back if isn't working well.