Knowing the size of your current drive and of your three partitions would help. However I will assume that if you buy a second hard drive, it will have at least the same capacity as your actual drive.
If you are going to switch from a one-drive to a two-drives configuration, I suggest the following (AKA Long answer):
- Copy your shared storage partition on the external drive using your favourite tool (KDE Partition Manager might already be installed on your Kubuntu system). Do not make the partition fill the disk: keep enough space after the partition for the next step. I suggest that this space equals the sum of the size of your system partitions; you will be able to change it later if necessary.
- As it has already been suggested, make backups for both of your systems, either by keeping only data and settings (fast) or cloning and compressing the full partition image (slow). You might want to look here for more details.
- Once you've made sure your shared storage partition has made it safely to the new drive (have a look inside to be sure), you may remove it from the first drive by deleting the corresponding partition.
- Expand and rearrange the partitions for your systems on the first drive if desired. However, be very careful not to move the starting point of the Windows partition, since that might break the bootloader (it can be fixed but avoiding it is safer).
- Make sure both systems recognize the partition and set the new partition to mount automatically on startup (you probably did that before with your current setup).
If there is anything else left broken, try to fix it or leave a comment here so we can help you complete the transition. I hope this method works for you.
