You have to get used to Puppy - it does not automatically mount your device. To do this you can go to the dock at the bottom of the screen and select the "drive mounter" icon. This will bring up a small screen which will have selections for Optical, USB. etc. Clicking on one of these selections will show you which devices are connected and a button to Mount. When you click Mount, if it is a storage device, the file manager opens and your files are shown.
Sometimes you need to select a file from an external device while working in an application. If you don't have the device already mounted, you cannot select the file. If it is mounted, the file can be located in "mnt" (dev)