My laptop has a 512GB ssd with windows 10 pre-installed in it. I was wondering if it is possible to put Ubuntu in a separate(external) hard drive with 1TB storage and use only when the hard-drive is connected to the laptop. Also I just want to create a partition on the HDD, say 200GB for Ubuntu and the rest for larger files like games or editing software that I can access using windows.
Yes it is possible. Even I have Ubuntu installed on an external storage device! At the installation, instead of Erasing the Disk, click "Something Else". You'll be able to manually create partitions on your external hard drive. From there, you should delete existing partitions on the drive (be careful no to touch your interal SSD here!), then you can make a 200GB ext4 partition (mount point should be
/ (root), a custom sized swap partition (I recommend 4GB), and the rest of the space can be created as an NTFS partition (readable by both Windows and Ubuntu). Just make sure you change the bootloader location dropdown to the name of your external hard drive.
If my answer was not clear enough or you need help, please comment and I will get back to you ASAP!
EDIT: If you are using UEFI, you will also need to make an EFI System Partition (ESP) of (around) 500MB. This is not needed in BIOS; only UEFI needs ESP.