Problems connecting with a WD external HD

Hi, I have a Macbook Pro and I can't add files on my Western Digital external hard drive. I can remove files but I can't transfer any files into it. My other hard drives work perfectly. I connected my friend's WD and the same thing happened, I couldn't transfer files into it as well. I was able to transfer files in and out of it using another laptop though. So is it just a problem with Western Digitals? Is there something I can do?

How is the external drive formatted? If it is NTFS, you will be able read from it, but there is no native support for writing to NTFS drives in OS X. There are third party drives you can get to add write support for NTFS formatted drives.

Further if you are going to use it with the MacBook only, format Mac OS Extended (Journalled) go into Partition, and partition if only the one partition and then into Options and make sure it is set to GUID.