can only copy about 1/2 files from portable hard drive

I'm trying to copy files FROM my portable hard drive (formatted in Windows NT Filesystem) to my Mac. I can copy about 1/2 the files from any given folder before getting error message, "The operation can’t be completed because an unexpected error occurred (error code -50)".

I use this portable hard drive to save files from my and it saves my Mac files there OK. I want to keep it formatted that way cause I use it for my pc too.

I know you're not supposed to be able to write to such a hard drive, but it copies files from the Mac there OK, and I can open files there OK

It seems like you should be able to copy files FROM the hard driver, but it doesn't do it. Help!