Partition error on external hard drive

Hi there,

I created two partitions on an external drive which seemed to work fine. After transferring some data onto it I decided I should create a third partition. I thought this would be possible without deleting the data but every time I try it, it gives me a partition error saying that it is 'unable to mount disk'. I'm currently copying the data back off again which is taking a while as it's 300GB but wondered if anyone has had this issue before and was able to resolve it? Otherwise I'll have to format it and start again but would find it useful if I was able to add/remove partitions at a later stage without having to format the disk and start from scratch.

Thanks for any help anyone can give 🙂