1. Plug (Plug-and-Play) USB an advantage compared to other traditional interface is plug and play implementation, Plug and Play (Plug-and-Play) can also be called hot-swappable (Hot Plugging ). For users, first insert the USB peripherals, automatically or manually install the driver, he should be able to use this peripheral, which is very user-friendly experience. If the operating system prompts you to restart the computer, indicating that the peripheral is not plug and play, from the strict sense, this is not a USB peripherals. If you tell the user it should be, would be deceiving users.
For Microsoft, if not plug, you can not get WHQL certified. As for the USB IF (USB Implementers Forum), if not plug, you can not get USB certification, can not use the USB logo. The logo in the USB's official website www.usb.org see (below article is also available), due after the USB2.0 specification was released, USB IF changed the original logo, some manufacturers still use the original logo, which is actually also deceiving users.
USB peripherals are many, many of which have similar characteristics peripherals, these peripherals are classified as a certain type, such as a keyboard, mouse, etc. for HID class audio products for the Audio category, hard disk, CD-ROM, Flash for Mass Storage class. Microsoft provides some commonly used class of class driver, if a peripheral designed to meet the specifications of the class, at the first insertion PC, the operating system can automatically recognize and automatically invoke the class drivers, which is called automatically installation. Currently Windows class driver can support only HID, Audio and Mass Storage, in addition to the manufacturer must provide their own drivers. Unfortunately, USB to serial applications fall into this situation, which requires so-called manual installation. Manually install there are two possibilities, one is at first inserted USB, the system will prompt the user to install the driver, you can follow the prompts to install the pop-up window, then the user may be required to do some searching, some users may find such is not easy; it is another way to provide an installer, all relevant applications / drivers packaged together, one-time installation is complete. If the driver does not get WHQL certification, the installation process requires the user to use the mouse to click on a second "continue" and "completed"; and if the driver get WHQL certification, you do not need too much trouble, which can also be called a quiet installation (Silent Install).

2, USB multi-serial protocol (UMP) when installing the USB to serial devices, users find that there will be two steps, one at the interface (and serial), there is a USB device, and the other is USB multiport serial protocol (UMP). In fact, this is correct, the first USB to serial device is used to simulate a serial (COM), it is a virtual COM port; secondly it is a USB to serial bridge, it must follow the USB multiport serial protocols.
In peripheral "Device Description" (Device Description), this device is a USB to serial bridge, peripheral access each time, the operating system will read the "Device Description" to understand what access equipment, and add it to the "Device Manager" (Device Manager). This is the first step, after the operating system knows that the peripheral is to achieve a virtual serial port, it will be added in the COM USB devices.
There are some applications, only the emergence of a virtual serial port, but no USB multiport serial protocols. In fact, this is made in the drivers hands, shielding a very critical first step, the report did not attribute to the operating system of the device. This may seem simple, but there is no possibility to obtain WHQL, USB can not get certified.

3,500mA USB bus-powered mode specification specifies a USB root hub (root hub) can provide up to 500mA, this area is to reduce the burden on the PC, on the other hand is to protect the USB host controller (Host Controller), currently the USB host controller is often integrated in Southbridge. In theory, any USB Hub should use a power switch to prevent over-current, but there are at least 10% of the board does not use the power switch or using some low performance component to save costs.
When a peripheral requires more than 500mA of current, you need to use a self-powered mode. This application is a USB hard disk bottleneck, but also a challenge. 2.5 inches above the hard disk itself requires more than 500mA, it must be self-powered, self-powered battery can be used, you can use voltage source, this is not the ideal method for mobile devices. There is a compromise approach is to connect two USB ports, you can get twice the current -1A. This is not the way, the same can not WHQL and USB certification. Operation is also very troublesome, you must first connect the power supply port, connect the device port, and so on.
The good news is increasing the size of the hard drive miniaturization, power consumption growing province. 1.8,1.5,1 inch drives have been widely used, more stable performance, these drives within 300mA of power consumption, if the USB controller chip power consumption can be controlled within 200mA of, you can realize by bus powered by. TI's TUSB6250 is the industry's first chip to achieve a bus-powered USB2.0 high speed (480Mbps) transfer ATA / ATAPI solution.

4. Conflict USB and ATA / ATAPI USB to ATA / ATAPI bridge controller, there are some inherent conflict, because the sides of the bridge are trying to control the bridge itself, such as USB and ATA / ATAPI bridge are yearning to send or receive data you can call it competition, which can not be solved when the bridge priority, then transfer data stopped. More serious is the bridge can not put this situation and notify the PC hard disk, PC machine and still continue to try hard, but did not prompt an error message to the user, the user will be very confused and do not know what happened. This time in addition to the shutdown to restart the computer, no other way.
The USB Mass Storage Class specification summed total of 13 different conflicts that can not be solved from the standard itself, only with hardware or software methods to solve. TI's TUSB6250 is a good deal and solve this problem, in fact, increased competition for arbitration to resolve the judgment of priority, at the same time allow only one control logic to control the bus, not clogging the bus.