Moving Folders OFF my iMac HD to another external HD to open space on my iMac HD.

Do the copy (not move) of the folders in your operating system, copy (not move) the entire "tree" of folders to the new drive, without make any changes, just copy and paste. Then point the LrC catalog to the new location of the folders (also called re-connecting), following the instructions here. Use the "Update Folder Location" command. Please see Figure 4 in particular for re-connecting the entire "tree" of folders in one action, right click on the parent folder and select "Update Folder Location" and then select the new parent folder on the new drive, which re-connects the Parent folder and all subfolders will re-connect as well.

Once you are sure everything is working properly in LrC from the new drive, you can delete the photos on the interal drive.

I do that kind of thing in one of two ways, but I would only do it one way to avoid potential corruption issues.

The potentially problematic way is to go to the Folders Panel, select any number of folders and then just drag and drop them to new locations. If your target drive has already been located by Lightroom, it'll be there in your list and you can easily drag and drop. The issue with this is that it's a move, not a copy. So if the transfer gets interrupted by power outage or drive cable disconnect or anything else, you might get corruption and lose data. So I don't do that very often, across disks. If I'm doing it on the same disk, and just moving folders in the logical hierarchy, then I'm not as concerned because the data's not physically going anywhere.

The better way would be to make a copy of the folders you want to move in Finder first, so that you have two copies. Then go to Lightroom and Right Click on the folders you want to move, and select Update Folder Location.... Then navigate to the additional hard drive where you want them to live, and select the appropriate folder.

If you're moving a lot of these folders, then I would nest them all under a parent folder first and find that new parent folder which will update all the subfolders at once. That's a little more complicated, but I can explain if you want.