What format is the drive? If you used it out of the box, it is probably in fat32 (maybe exfat). Both have limitations in file name length - 11 or 12 characters.
If you do as Fravin suggests be aware that the disk won't be useable on a Windows System, if you use it on both systems. GUID partition with exfat fomat is not readable on Windows.
If it is formatted as HFS is it possible it has a security lock of some sort on it??