Lets say I am zoomed in, 500%, and I want to go beyond the edge, right edge, and when I try to drag it using the hand tool it doesn't work and the scrollbar doesn't work.
There are additional plug ins from Adobe you can install. If you install the optional "Always Overscroll" plug in, you can always scroll past the canvas.
In addition, if you are in Full Screen Mode (
View > Screen Mode or tap the F key) you can also always move paste the canvas.
In normal view, you can't overscroll until the image is zoomed larger than the available view.
PS CC 2014 adds overscroll in normal view as an interface preference (Preferences > Interface : Options > Overscroll).
The most recent versions of Photoshop CC seem to have moved the Overscroll option to the "Tools" panel within preferences, rather than "Interface."