How do I merge Photos/iPhoto libraries housed on external HDD into current iCloud Photos library?

For what it's worth I did a study between using Photos and PowerPhotos for importing a library into an empty Photos library and this is what I got:

I ran a number of tests with a 3119 photo, 72 video and 97 keyword Photos library with keywords, captions, keywords and locations.  I merged it into an empty library using Photos and PowerPhotos.  This is what I found was imported by each;

PowerPhotos:           Photos:

Albums***                                          -----------

Smart Albums (as regular albums)** -----------

Folders Containing Albums -----------

Captions                                             Captions

Titles                                                   Titles

Keywords* Keywords (some but not all: …

…see Note *)

Original images                                  Original images

Edited images                                     Edited images

---------------                                        Locations

Favorites                                             Favorites

Can detect and exclude                      Duplicates

duplicates upon import or not

Neither method could import/merge projects. 

Both methods could import older iPhoto libraries into a Photos library when the iPhoto library couldn't be migrated into a Photos library. 

Notes:

*The Photos app only imported 84 keywords our of 97. PowerPhotos merged 147 keywords. Photos only imported keywords that were embedded in the original file and not those added by Photos. PowerPhotos imported all of them.

**The original library had 90 Smart Albums. All were merged by PowerPhotos but, as indicated above, the Smart albums were brought over as regular albums. No album of any kind were imported by Photos. Also only PowerPhotos imported folders.

The original library had 34 regular albums and 1 folder with some nested albums. PowerPhotos merger all of them successfully.

The original library had 72 videos. Photos imported only 57. PowerPhoto got all 72 in its merge.

The tip is: Comparison of PowerPhotos vs Photos in Merging one Photos Library into Another - Apple Community

What OS is your Mac running?

First, how is your external drive formatted? To avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. The drive must be connected directly to the Mac by cable, not networked, clouded, NASed, etc. Additionally, the drive can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:

Move your Photos library to save space on your Mac           

If this drive is in a an incompatible format, stop running Photos with it immediately!  A Photos Library can sit on an incompatible drive, but running it may corrupt the database.

Your Photos Library is not " housed in iCloud;," rather, it is on your Mac. A copy of your System Library is kept at iCloud Photos at iCloud.com. It's a copy, and if your local Library changes, the copy at iCloud Photos will copy the change. Do you use Optimize Storage with Photos on your Mac?

Be sure to keep these older Libraries untouched, and work with copies. To be used and merged, the copied Libraries will have to be upgraded to work with your newer OS, so there are things that might go wrong, and you'll want to have the originals. You will need enough room to hold the larger Library.

Using Photos' File>Import command, you can bring in pictures from other Libraries, but you lose the organization of albums and folders. As Yer_Man has said, the best way to combine Libraries is with the trusted 3rd party app PowerPhotos ($40.)  Fat Software started with iPhoto, so they have lots of experience and are highly dependable. PowerPhotos has lost of useful tools in addition to merging Libraries.

Let us know if you need more detail…

The best way to do this while preserving metadata and avoiding duplicates is with PowerPhotos:

https://www.fatcatsoftware.com/powerphotos/

Yer_Man wrote:

The best way to do this while preserving metadata and avoiding duplicates is with PowerPhotos:

https://www.fatcatsoftware.com/powerphotos/

Yer_Man wrote: …I'm not convinced that you'll preserve as much of the metadata as you require, particularly from the older library. However, a little trial and error will help there, no?

I did some trial and error. It looks like when you use Photos' File>Import to bring in pictures from another Library you get metadata with edits and crops. Titles, Captions, Keywords and original file names are the same, and without going through every line in Exif, it looks like all the same stuff. But, as we said, you don't get albums and folders.

Old Toad's analysis is here, and it seems to apply to Tahoe:

Comparison of PowerPhotos vs Photos in Me… - Apple Community

A current version of Photos can import from older libraries, but I'm not convinced that you'll preserve as much of the metadata as you require, particularly from the older library. However, a little trial and error will help there, no?