Win 10 when installing should allow you to go to the advanced mode to access the hard drive and allow you to delete all partitions then create 1 single partition. Be advised though, there will be another partition created for the recovery but it only uses a small amount, (1-2GB). The HDD should not be connected at the time you are doing this. Only connect after your OS has been installed. Here are 2 videos you can watch to help you out.

1. HERE

2. HERE