Sorry for my late reply.
By multiple libraries, I mean, saving a library to an external hard drive with photos in it. This way, I have those photos in one place.
I want to be able to import those photos from the library again and export them for safe keeping. As a result, I will have the exported library of photos on the external hard drive, and I will also have those same photos that I exported as Jpegs on an internal hard drive.
I'm still slightly confused by your explanation, but let me take a minute to talk about how Photos works. You did NOT say you are using the Photos app, but from your description and terms like "import," "export," etc., I think you are using Photos. If you are not, you can stop reading here as the next bit talks about how Photos works internally.
Photos has an "import" function that has two options. One option, the default, is that the actual image file is brought into Photos as an element added to a large database. The import then reads all of the metadata about the image (where, when, settings, etc) and stores that information in one table and then stores the actual image data in a separate table in what is called a "blob" format. (As an aside, a "blob" is a Binary Large OBject, a BLOB.) So, when you look at the image in Photos, it retrieves the blob and marries it to the metadata again so you see the image and can manipulate it as you want/need to (edit, crop, change colors, etc.). Those changes are also stored in the Photos app in a third table in the database, with indexes for the order in which you made the changes. Note that the original image is kept, along with a list of changes you made. So, after saving the changes, the next time you open that image in Photos, the original image is opened, metadata merged and then all of the edits you made are applied to the image, in the order you made them, and then the image is displayed to you. All of that takes place very, very quickly and you should not notice it happening unless you made thousands and thousands of edits.
The second option for import is to leave the file on the drive where it is and only store the location of that file in the database in Photos. When you open the image in this case, it opens the external file from that location, reads the metadata and stores it in the database and presents it to you as an image. When you edit that image, it stores the edits in the database, but I think when you save the image back to the system and close Photos it actually overwrites the original image file with the edited one. I don't use that method myself because I like the idea of having the original image available to me just in case I want to change the editing, so I am not totally sure how it handles the original if it is NOT imported into the database as a blob. I think you lose the original image as it is overwritten by the edited one. Maybe someone who uses this import option will chime in and say what happens. I also don't know if the edits can be reversed using this method. In the first, default, option, the edits can always be reversed as the original file is still stored in the database.
When you "export" from Photos, what happens is that the blob is retrieved, edits applied, metadata reattached and a new file created with all of the information to go along with it. Where it is saved and what it is named is up to you at the time of export, so if you save it to where it came from and use the same name, the system will ask if you want to overwrite what is there, which, again, is up to you. I don't know what "export" means for an image not imported as a blob, as there is nothing actually in the database to export. Maybe, and this is speculation on my part, it reads in the file from the disk location, then offers it to be "exported" to wherever you want. That action seems pointless as you can just copy the file from where it is to where you want it to be, but again, I don't use that method, so I just don't know.
If you use the second option, not importing the image to a blob format, then it is imperative that you NOT move or change the file actually on the drive. If you do change the file on the drive (rename, move, delete), Photos will NOT be able to find it any more and will produce an error to that effect.
As you can imagine, as images are imported a blobs to the database, it grows very large. My current photoslibrary file has about 12,000 images and videos and sits at 143 GB. That library can be anywhere on any drive, internal or external. I have it on my internal drive because I use my laptop alone a lot of the time and want the library with me. I have multiple copies of that library as backup, on both direct-attached drives and on a networked RAID drive as well as being synced to my iCloud account for storage in the cloud. And Time Machine backs it up as part of the regular backup process, too. I have had the unfortunate circumstance of losing the internal drive and a backup drive on the same day, losing about 8000 irreplaceable images, so now I have multiple copies in multiple locations and multiple media so that the probability of losing them all again is greatly reduced.
OK, with that background out of the way, what is it that you REALLY want to do? Your statement:
I want to be able to import those photos from the library again and export them for safe keeping. As a result, I will have the exported library of photos on the external hard drive, and I will also have those same photos that I exported as Jpegs on an internal hard drive.
mixes up the technical terms export and import as Photos works, so I don't really know what you want to do. There is NO need to repeat the import/export process to work with Photos. Just use the default import, get the image into Photos, edit as you want, let TM do the backup, or make extra copies of the photoslibrary file as additional backup. That takes care of the "safe keeping." Copying the photoslibrary to the external drive will let you have a second copy there. You don't really need to keep the jpeg files on the internal drive unless you use the second import method, in which case you MUST keep the images where they were when imported or Photos will lose them. Any time you want a jpeg file, you can export THAT image from Photos. And if you use the second option, the jpeg files are already on the drive, so no need to export them.
Hope some that helps somebody.
