Instead of the external proprietary drives of the Xbox 360, the Xbox one will support any external storage device.
Any storage device than can connect to the USB 3.0 slots imbedded into the Microsoft's new console, will work as long as it performs the same actions as the internal drive. This news comes as a surprise considering the Xbox 360 would only support the grossly expensive hard drives sold and designed by Microsoft. They would later add USB storage support but limit the capacity initially to 16GB and later update it support 32GB.
As for the internal storage, if you don't want to place a large storage device above your console or just want to replace the internal storage, owners cannot do so.
The ability to add external storage to your Xbox One, considering games now require a mandatory install, far outweigh the problems of a non-removable internal storage device. If games did not require the mandatory install, the 500 GB internal hard drive size fairs reasonable for even the most active player. Storing all the videos, games and screenshots taken on your Xbox One, will definitely fill up quicker than expected.
Source: Engadget
