From Sigma:
Thank you for purchasing and using our products.
We are pleased to inform you the SIGMA 120-300mm f/2.8 DG OS HSM F/CANON Firmware Update is now available.
This firmware update allows you to use the SIGMA USB DOCK with the SIGMA 120-300mm f/2.8 DG OS HSM F/CANON. If you have the products mentioned below, please start up SIGMA Optimization Pro and then operate lens firmware update.
Applicable products: SIGMA 120-300mm f/2.8 DG OS HSM S013 F/CANON
Benefits of this firmware update:
Please update to SIGMA Optimization Pro Ver1.2 before operating firmware update of SIGMA 120-300mm f/2.8 DG OS HSM S013 F/CANON.
SIGMA Optimization Pro Download Page
Note: Apparently, blogger/photographer JerseyShooter had a big hand in alerting Sigma to the need for this firmware update. Check out his blog for more info.
B&H carries the Sigma 120-300mm f/2.8 DG OS HSM lens and the Sigma USB Dock.