How to transfer photos from OS X Yosemite to USB?

I have version 10.10.5. I recently imported the photos on my iphone to the "Photos" app on my mac. I want to transfer them to my storage drive (name: MY Passport) but i don't know how.
Looking at some answers online, they just say to drag them but when i do that, there's a symbol that appears that i'm not allow to do that. When i try to "export" them, an error message appears saying:

"You can't save the file "XXXXX" because the volume "Macintosh HD" is read only."

I really don't know what to do. This is my first mac (MacBook Air) and i'm pretty bummed out that many easy tasks on windows are so complicated on OS, but also I'm not so computer-literate either.

I have version 10.10.5. I recently imported the photos on my iphone to the "Photos" app on my mac. I want to transfer them to my storage drive (name: MY Passport) but i don't know how.
Looking at some answers online, they just say to drag them but when i do that, there's a symbol that appears that i'm not allow to do that. When i try to "export" them, an error message appears saying:

"You can't save the file "XXXXX" because the volume "Macintosh HD" is read only."

I really don't know what to do. This is my first mac (MacBook Air) and i'm pretty bummed out that many easy tasks on windows are so complicated on OS, but also I'm not so computer-literate either.

Thank you in advance for your help. You should be able to drag them out of Photo's window onto the My Passport's icon (if that is showing on the Desktop.

You should be able to select Export from Photo's File menu and then select My Passport from the Export pane.

You should be able to open the My Passport in a Finder window and then drag the pictures in that window.

The Macintosh HD should not be read only. That you saved the pictures to the hard drive in Photos suggests that it has not always been read only.

You should be able to click on the image below and drag it out of the browser window on to your Desktop. What happens when you try… Curious… What format did we format the Passport to?

Curious… What format did we format the Passport to?

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