Windows Network
Windows uses a network file sharing protocol called SMB (aka Samba). Ubuntu comes with Samba to communicate with other Windows computer in the same network. This means if you have other computers running Windows in your home network, and you set up these Windows computers to share files and folders over the home network, Ubuntu can access those files and folders. It appears you don't have any Windows computer in your home, or you have not set them up to share files. Hence you get the error when you click on Windows Network icon under Other Locations. Other locations means other computers in your home this context.
Files in the "Other Drive"
Say you had two houses. You demolished one house and built a new one in it's place. Then you go to the 2nd house and find it has all the old furniture inside! Those are your furniture in the second house.
You had two drives, an SSD and a HDD. When you installed Ubuntu in the SSD, the installation process formatted the SSD. At that time the installation warned you that it will delete everything in the SSD. Since the HDD is a different physical drive Ubuntu installation did not touch it.
Note. Windows confuse you with C: drive and D: drive, where these are actually partitions. The D: drive can be a partition within the SSD or a partition within the HDD. If D: drive was in the SSD it would have been erased during the installation. If the D: drive is in the HDD it won't be erased, as it seems to be in your case.
Therefore, when you look inside the HDD and find some furniture, I mean files.
The NTFS partition is Read Only
This is because by default Windows uses Fast Startup, which does not properly shut down the computer (and the drives) when you shut down your computer. I assume you did not disable this feature in Windows before you shutdown Windows for the last time (before you installed Ubuntu).
Ubuntu sees the partition in your HDD as "dirty" and opens it in Read Only mode. Usually we recommend you fix problems created by Windows by booting Windows. In this case that is not an option. However, you are not the first (or the last) person to be in this position. This site has many questions and answers on this topic. These two may help:
Fix corrupt NTFS partition without Windows
How to correctly fix a "dirty" NTFS partition without using chkdsk
Alternate solution: Reformat and Reuse
First copy any files and folder you want to save from the HDD, in an external drive.
Then format the partition in the HDD (it looks like a single partition in the HDD) using the ext4
format, the native partition type used by Ubuntu. Since you don't have Windows in this computer, I recommend you do not use the NFTS format. This is because, if and when an NTFS formatted partition gets corrupt, you may need to boot Windows to fix it.
See this answer for more on using the second HDD:
How to enable full permissions on internal secondary HDD?
Hope this helps