Uploading from external HD

You do mean Photos library on the EHD, right?

If so then you would have to designate the library on the EHD as the System Photos Library and enable iCloud Library. It will then upload all of the photo to the iCloud Library and download all of the photos in the iCloud Library to the EHD Library.

Once done set the library on the iMac to the System Photos Library and enable iCloud Library. The iCloud Library and that library will then sync so they are the same.

HOWEVER, if you just want to get the photos into your iMac library, you're running Monterey and both libraries are V7 you can import the EHD library into your iMac library.

If you're on another system and Photos version you cana use the paid version of PowerPhotos to merge the EHD library into your iMac Library.

iCloud is not off computer storage. It's a syncing feature so you can have the exact same library on multiple devices, iMacs, iPads and iPhones. Any photo in the iCloud photo library will be mirrored on your iMac.

Are you using iPhoto or Photos? What system are you using?

That's correct.

What system and iPhoto or Photos version are you running?

So you have Photos 7. You should be able to import the photo in the library on the EHD into your MBP library. That would be the quickest way to get the photos into your primary library.