There are 3 server ranges at OVH. Enterprise, Hosting, Infrastructure.

Enterprise servers are used primarily as a multi-purpose business server.
Hosting servers are commonly used for web agencies, hosting providers and value added resellers.
Infrastructure servers are aimed for outsourcing and disaster recovery plans.

The servers come in a pre-defined configuration, typically they are not upgradable in any aspect, there is two exception to this case.

The HG servers within the infrastructure range does allow an custom server order, this means the server can be tailored to your requirements and disks within the server can be added and removed as needed.

The server itself has two USB ports, this means you is able to add USB keys storage to your server. Each server can have up to 2 additional USB keys. The USB key is 3.0, however the speed of the USB will also be dependent on the USB port. The USB keys comes in the following sizes: 16GB, 32GB, 64GB, 128GB and 256GB.

You can find more information about the USB key here.