Photos App deleted all of my photos during a transfer to an external drive
As Tetsujin said, I agree that we cannot help much with the information provided.  But I'd be surprised if they were gone (reasoning at end).  However I will make an assumption that you have a photo library in /Users/username/Pictures/Photos in Library.photoslibrary  

I would right click on the "Photos in Library.photoslibrary" file - select "show package contents."  Go the the folder "Masters" and see if you can find your pictures in there.  Do this for both where you copied the folder to and where you copied the folder from. 

You can also search in here for something like *.JPG or *.JPEG or type JPG and select kind: JPEG Image.  

Anyways, if the images are no longer stored in the package contents of either where you copied it from or where you are copying them to, unfortunately without significantly more advanced methods, they are most likely gone.  However most protocols do a write, verification, then delete, so I'd expect that the file is just not configured properly for Photos to read and your pictures are still there, just the Photos app isn't seeing them.  For example in terminal when you execute a "mv" command you are actually doing: 

rm -f destination_path &amp;&amp; \
cp -pRP source_file destination &amp;&amp; \
rm -rf source_file


Notice the &amp;&amp;.  The destination file is deleted no matter what if it's already there (overwriting a file, so if the copy fails that file is still removed).  The copy, however, must be successful before the source file is deleted.  It doesn't follow symbolic links, but Photos used Hardlinks to migrate from iPhotos to Photos to save space.  So this is not an issue.