Check the /var/adm/messages file for failures to enumerate the device. For enumeration failures, possibly, insert the USB hub or remove a hub and connect it directly to a root USB hub.
If USB devices are added or removed when the system is down, you must perform a reconfiguration boot.
ok boot -r
If you have problems accessing a device that was connected while the system is running, try the following command:
Do not move devices around if the system has been powered down by a suspend operation. For more information, see SPARC: USB Power Management.
If a device has been hot removed while in use by applications and is no longer available, then stop the applications. Use the prtconf command to see whether the device node has been removed.