How do I transfer photos from mac to external hard drive please

Connect the HDD to the MBA and 'drag and drop'.

Ciao.

This answer may need to be expanded a bit. If your photos are in iPhoto and you are moving iPhoto's library to an external drive, once you've dragged the library to the external drive you'll need to let iPhoto know the library has moved. Hold the option key and double-click iPhoto (or click on the iPhoto icon on the Dock). You'll get a fileselector menu to select the library - point it to the external drive. Once you've established that the copied library has copied correctly you may drag the library on your computer to the trash and empty the trash.

Warning: You do have a backup of your entire computer, right? Just in case something goes awry with the process having a backup is always a good idea.