I've found that while copying a large number of files from my NAS to an external hard drive over USB 3.0 unusual file system errors are introduced to the NTFS drive if any files in the list to be copied are modified before the data is written. The errors aren't apparent when the drive is connected to a Windows pc but when connected to my BSD computers the available drive space appears to be truncated and the directory structure is malformed. Since I'm often updating and editing multimedia files this can be a nuisance. The only fix I've found so far is to reformat the external drive from a Windows OS using their native format utility. This isn't a great solution for me since I don't usually have a Windows computer available. Borrowing a friend's computer to format a large external drive is no fun. Often it's easier to sell the offending drive and buy another.