I just installed a USB 3.0 HDD Dock, and tried copying a large block of files to it using DO 10. However, partway through the process I got an error saying the path could not be found, and the drive was no longer seen by the system. Rebooting the computer brings the drive back, but continuing to copy files with DO results in the same behavior - after copying a number of files it stops again and the drive letter disappears from the system. I verified that all files prior to the error were copied properly. Here's the thing - if I use Windows Explorer, this error does not happen, so I know there is not a problem with the drive. This also happens whether or not my anti-virus program is running. Relevent info: Windows 7 64 bit, 16GB RAM, USB 3.0, Directory Opus 10 64 bit version. All hardware drivers are current. Thanks for any help.
Opus itself would not tell the drive to dismount and essentially just opens files and writes data to them.
Maybe something Opus does slightly differently to Explorer is triggering a bug in the USB drivers/firmware/etc. (Most things are only tested with Explorer, so they'll usually work with that, of course.)
Changing the Copy Buffer Size which Opus uses will sometimes help with buggy USB devices, so that may be worth a try. (Preferences / Miscellaneous / Advanced: copy_buffer_size)
You can also set up a button in Opus that copies files using the shell (i.e. the same way Explorer does) as a workaround: Copy files via the shell (Windows Explorer)
Thanks for the quick response! Would you suggest increasing or decreasing the buffer size?
Both. 
There's no right value that works with everything, unfortunately, so it may take some trial and error. (Or course, it's also possible that it won't make a difference.)
As a quick test, I'd try 16, 32, 128 and 256. If none of those values seems to make a difference then it's probably not worth trying others. (If they give less frequent disconnects, without solving the problem completely, then it might be worth trying similar values, but if it makes no difference then it's probably not the buffer size that's causing the problem.)
Well, setting the buffer to 256 seems to have worked so far, at least I was able to do a full backup without interruption this time. Thanks for the advice !!!