MacBook Pro :: Errors When Copying Files To External HDs?

I have two WD Elements external HDs. I use a Belkin powered 2-port USB hub to connect them to my Airport Extreme (used as a server). I started out with one of them, a 1.5 TB, and purchased the second one when the first started to get really full. I use them exclusively for storing music files. My plan was to move files from the  nearly full drive to the new one. I began moving files, folder by folder (a painstaking and time-consuming process) from the one drive to the other. 

The first problem that popped up was this: in the process of copying a folder to the new drive, an error message appeared saying that the "operation could not be completed because you do not have access to some of the items." I in fact have full custom access to both drives, and am the only one who uses them. Another message I would sometimes get would be, "[file name] could not be moved because its name contains too many characters." 

I found this annoying but bearable until it started happening more frequently. What's more, when a folder failed to copy completely, that would leave me with an identically named but incomplete folder in the second location. An attempt to delete it would be met by another error message, "[folder name] could not be deleted because [folder or file name] is in use.