Internal storage devices reside inside your PC. Once installed, they become an integral part of your machine, and due to the complex process of installing, you would definitely hesitate before removing them again. Hence if something goes wrong with an internal storage device, more often than not, you need to take it out of your PC system to have it fixed or to have it replaced. For data back-up, an internal storage device is not a good option since you're most probably backing up files coming from your own PC. Having it connected to your PC most of the time would render it vulnerable to virus and other malicious attacks coming from online predators.
On the other hand, an external storage device gives you exactly the opposite of that limitation of internal storage devices. You can bring this kind of storage device anywhere, and the only time it stays connected to your main machine is when you are using it. And since it is portable and can universally connect to all PCs which support it. External storage devices are a great accessory for bringing large files to where ever you want to bring them.
In terms of speed, internal storage devices though beat external devices. Since external storage devices rely on another technology/facility to connect it to your computer, transfer speed is severely affected. Internal storage devices download and upload files to your PC faster than the external devices. This is due to the fact that internal storage devices become part of the system once it is installed.
On the sharing aspect, external storage devices definitely beat internal storage. You can easily share your files with your friends and contacts by simply connecting the device to their machines. And since most external storage devices now work through USB port connectivity, you are pretty much assured that you can easily transfer your files to your friends'/contacts' PCs in a snap.
Price wise, external storage devices are at the high end as compared to internal storage device. Manufacturers of external storage get tons of mark-ups to regain at the same profit. This is due to the fact that external storage devices require special casings that could prevent your device from malfunctioning since it helps in radiating the heat emitted by the device away from the drive itself.
When choosing the right storage device for you, carefully weigh-in the pros and cons. Choose the one that best suit your needs and your machine's capability. External storage devices may be cheap in price but you'd never know how capable it is in efficiently doing its purpose.