1, that depends on where the catalogue is located
2, Unless you added the photos from your Photos program no
3, All my files is all the files on all connected drives
4, Again that depends on where your Library or catalogue is stored
Just move both the Photos library and your Lightroom catalogue to the external drive so you know for sure where everything is. See here How do I move a catalog? https://helpx.adobe.com/lightroom/kb/catalog-faq-lightroom.html
To move your Photos library just quit Photos and drag the Photos Library from your Pictures folder to the external drive then trash the one in Pictures and empty the trash then double click on the one on your external drive to make it the default Library
It also depends on how you import pictures to lightroom:
When you import images to Lightroom, Lightroom gives you four different ways to do this, and they're all at the top of the Import window. You select which one you want by clicking on it. From left to right, the options are:
Copy as DNG. This converts your RAW images to DNG (Adobe's digital negative) file format as the files are imported. The RAW files are left on the card. This has the tradeoff of not requiring sidecar .xmp files to store the edit history of the file, but can lose you proprietary metadata of the specific RAW format you're using.
Copy. This copies the photo files from where they are into wherever you tell Lightroom to put them as you import. This will leave all the files on the card alone, which can be useful if there's a problem with the import, you want to keep the card for backup, etc.
Move. This moves the photos from where they are into wherever you tell Lightroom to put them as you import, which will automatically remove the files from the card.
Add. This just adds the photos to the Lightroom catalog where they're resident. This can be handy if you like to copy the files from the card to where you want them to live on your OSX box by hand before doing the import.
You don't have to start a new question no answer if final until you are satisfied. As to your question about deletions, as I said it depends on how you imported those pictures to Photos or to Lightroom. I have no idea what is on your "external drive". If your Photos Library is on the external drive then yes those photos will be deleted from the external drive. What is on this external drive is it just loose pictures or what?
I suggest you only use one photos program too is there a reason your friend is using both?
I don't have enough information to answer your question about deletions at this point. Where are your Photos Library and Lightroom catalogue stored on the external drive or the main drive? Do you have Photos set to copy pictures to the Photos library when imported or to leave in place? I need more information.... you actually asked 4 questions... but I am willing to remote connect and look at this situation for you for an additional fee.
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