USB HDD Volume Shrink

I have a Seagate Free Agent GoFlex USB HDD 1.4TB which I have been using in the last 3-4 years for my backups. Presently I probably have about 300GB of backup data on it which I would prefer to keep

Four days ago I bought a new Asus K750JN laptop with windows 8.1x64 and as I just discovered to make an image of the OS (laptop came with no OS DVD) I must use a 16GB min USB. Furthermore it says that all data on the USB will be erased

I would have liked if possible to shrink the volume on my Seagate USB HDD to make two partitions: the root will be to copy my Asus OS and the other partition will be for my existing backups.

I assume that if I can shrink the Seagate existing volume I should be able to keep my existing data but if not I could transfer them to my desktop PC (long process but can be done)

I also assume the Asus utility which will copy the new OS to Seagate root partition will not erase the second partition

I could buy a 16 or 20GB new USB but since I have the large Seagate USB I think it will be waste of money

Any suggestions?