I'd suggest doing what Zardoc said. Log off, power down, remove device for this one time. Once you've got the device safely removed, I'd then make sure you have the latest release of USB Safely Remove. Version 4.5
was released just a few days ago. Next, I'd temporarily move whatever data is on the external HDD to another location (perhaps the internal HHD.) Then format the external HDD to give yourself a clean platform to work on. Reinstall your data and see if the problem is fixed. If you still can't safely remove the Samsung S2, it might be because the drive is going bad.