When you plug in a USB device, the device is immediately seen in the system's device hierarchy, as displayed in the prtconf(1M) command output. When you remove a USB device, the device is removed from the system's device hierarchy, unless the device is in use.
If the USB device is in use when it is removed, the hot-plug behavior is a little different. If a device is in use when it is unplugged, the device node remains, but the driver controlling this device stops all activity on the device. Any new I/O activity issued to this device is returned with an error.
In this situation, the system prompts you to plug in the original device. To recover from accidentally removing a busy USB device, do the following:
Plug the original device into the same port.
Stop the application that is using the device.
Remove the device.
The USB port remains unusable until the original device has been plugged in again. If the device is no longer available, the port remains unusable until the next reboot.
Data integrity might be impaired if you remove an active or open device. Always close the device before removing, except the console keyboard and mouse, which can be moved while active.