Check the /var/adm/messages file for failures to enumerate the device. In cases of failures that involve hubs, insert the USB hub or remove a hub and connect it directly to a root USB hub.
If you have problems accessing a device that was connected while the system is running, issue following command:
# devfsadm
Do not move devices around if the system has been powered down by a suspend operation. For more information, see USB Power Management.
If a device has been removed while in use by applications and is no longer available, stop the applications. Use the prtconf command to see whether the device node has been removed.