The above should cover what you need to do. Pay particular attention in step #2 Click here for a full list of all the Lightroom files. You will need to move ALL of these files and folders from the old system and old external HD to the same locations in the new system and new external HD.
You are also upgrading to a new version of LR, which requires additional steps to convert the old version catalog. You said, "I imported a folder from the old HD to new HD through LR on my new iMac." By importing this folder you've created a new catalog on the new system, which only contains the pictures in that folder. It is of no value if you want to move over your complete catalog from the old system and should be removed first before doing anything else:
1) Close LR and delete the contents of this folder on your new Mac system HD / Users / [your username] / Pictures / Lightroom /
2) Also delete the folder on the new external HD you imported into LR, but first make sure it is still on your old external HD. if NOT copy back onto the old HD into it's original folder location.
You will replace the above removed contents using the procedure outlined at the Lightroom Queen article link. As you can see things can get messy if the proper procedure is not followed when moving LR to a new system. In your case you are also upgrading LR, which adds another step to the migration process, but shouldn't cause any problems if you follow these instructions. Lightroom will prompt you that the old catalog needs to be upgraded the first time you start it after moving everything over. It will then convert your old catalog. What versions of LR are you using (OLD version, NEW version)...for example LR4.4.1 to LR5.5.