Thanks Linc for your advice. I will try and reformat the drive as ExFat, which I presume is an option in the disk utility?
It's strange though as I've had hard drives before where I haven't had to format them and I've inter-used with both Mac and PC? No probs will reformat.
Mac OS Extended format is only readable on OS X and you need third-party apps on Windows to read it.
To format your external disk:
1. Make a backup of your external disk files.
2. Open Disk Utility, select the external disk in the sidebar and go to Erase tab.
3. Choose exFAT as Format and select Apply.
4. Copy the data to the external disk