Cannot overwrite file: "because the item is in use"

Hi there - have much frustration trying to overwrite file from our Macs (OSX 10.9.5) to ReadyNAS 4TB.

If we have project files stored locally on the Mac, and want to back them up to the ReadyNAS, we want to overwrite the current files on the ReadyNAS. However, we often get an error message part way through copying saying "The operation can't be completed beacuse the item "xxx" is in use."

I think the problem is that if the main folder has a sub folder, it will not copy it across. We obviously have lots of sub-folders, and so this is really frustrating. It also happens when trying to delete a folder off the ReadyNAS - you have to continually delete the folder, as each time it will only delete one of the enclosed folders.

Any suggestions would be really appreciated!

Many thanks.

Ian