Copying HDD data to ext HDD - Wrong capacity

Hello,

I am trying to copy my windows C: data from my internal desktop hard drive to my external hard drive just to make sure I have a backup. I currently have 350gb of used space on my internal hard drive which I'm trying to copy onto the external hard drive however it's only copying 39gb instead of 350gb.

Internal desktop hard drive = NTFS
External hard drive = exfat

I have also formatted my external hard drive to NTFS but I get the same problem - This is very strange!

Would anyone be kind enough to help?

Thanks for your time.

Ben