You can add multiple USB devices to a virtual machine when the physical devices are connected to an ESXi host. USB passthrough technology supports adding USB devices, such as security dongles and mass storage devices to virtual machines that reside on the host to which the devices are connected.

When you attach a USB device to a physical host, the device is available only to virtual machines that reside on that host. The device cannot connect to virtual machines that reside on another host in the datacenter.

A USB device is available to only one virtual machine at a time. When a device is connected to a powered-on virtual machine, it is not available to connect to other virtual machines that run on the host. When you remove the active connection of a USB device from a virtual machine, it becomes available to connect to other virtual machines that run on the host.

Connecting a USB passthrough device to a virtual machine that runs on the ESXi host to which the device is physically attached requires an arbitrator, a controller, and a physical USB device or device hub.

USB Arbitrator

Manages connection requests and routes USB device traffic. The arbitrator is installed and enabled by default on ESXi hosts. It scans the host for USB devices and manages device connection among virtual machines that reside on the host. It routes device traffic to the correct virtual machine instance for delivery to the guest operating system. The arbitrator monitors the USB device and prevents other virtual machines from using it until you release it from the virtual machine it is connected to.

USB Controller

The USB hardware chip that provides USB function to the USB ports that it manages. The virtual USB Controller is the software virtualization of the USB host controller function in the virtual machine.

USB controller hardware and modules that support USB 3.0, 2.0, and USB 1.1 devices must exist on the host. Eight virtual USB controllers are available to each virtual machine. A controller must be present before you can add USB devices to the virtual computer.

The USB arbitrator can monitor a maximum of 15 USB controllers. Devices connected to controllers numbered 16 or greater are not available to the virtual machine.

USB Devices

You can add up to 20 USB devices to a virtual machine. This is the maximum number of devices supported for simultaneous connection to one virtual machine. The maximum number of USB devices supported on a single ESXi host for simultaneous connection to one or more virtual machines is also 20. For a list of supported USB devices, see the VMware knowledge base article at http://kb.vmware.com/kb/1021345. You can add USB 3.0 devices to Mac OSX guest operating system for VMware Fusion.