Why is my NTFS disk read-only on my MacBook?

I have been using an NTFS disk with my iMac to store and edits photos successfully - ie I can read off the disk and write to it. I want to use the NTFS disk with my MacBook but while I can read off the disk, I am unable to write to it and get a message saying it is "read only" Can anyone advise how I can change it to read and write?

Thanks.

[Re-Titled by Moderator]