Hard drive shows much smaller than is

I have two main 1TB SATA drives which are now designated as:
C: (Drive 3), and
D: (Drive 2),

and two old IDE drives I keep on for making redundant data backups,
E: (Drive 0) and
F: (Drive 1).

Prior to a disaster, Drive 2 was C: and Drive 3 was D:.

Yes, I have to be very careful over making certain I never presume that C: is 0 and so on...

I had W10(Insider Program) installed on Drive 2 (Prior to disaster, this was my C: drive). I had a major problem and had to reinstall W10 from a downloaded image. It refused to install on C: - Drive 2 that it was originally installed on - Windows could not reinitialize the deployment engine., so I installed it on D: - Drive 3.

The new installation is on Drive 3 that I have now re-designated as "C:", and re-designated Drive 2 as D:.

In order to wipe the D: drive clean from the old Windows installation* and the attempted new installation that failed because of the above cited error, I changed Properties:Security:Advanced:Owner of everything on it to Administrators and gave Administrators "Full Control". I then deleted everything from that drive, including all hidden files except for the drive ID.

I have two issues now -

1. If I create a text file on D: and then try to modify it, I can't. I am told I do not have permission, even though I am a member of the Administrators and Administrators has "Full Control".
2. The drive shows as 186.31GB; it is supposed to be a 1TB drive. I know I messed up as I previously saw the balance of space beyond the 186.31GB in "Disk Management" as something like unallocated space or the like. I tried to delete the volume thinking I could then allocate the entire space but I was definitely wrong. Is there something like the old-fashioned "low level format" that I need to do now so the system will work with the full amount of space available?

I have just enough knowledge to be a danger to myself; this isn't the first time I have reaped the rewards of my ignorance. I am thankful that there are folks out there like you reading this that actually know what you are doing to help me; Thank-You.

I appreciate your guidance!

* Either disk cleanup, system files, wasn't getting the job done entirely or some other reason, but as I still had the problem of being able to delete the failed Windows installation that was not erased upon failure, I came up with the change of ownership as a means of deleting everything off of the drive.