Help! Left Backup External Drive Connected While Reinstalling Windows... Lost My Data!

Getting ready to reinstall windows on my computer, I backed up everything on an external drive. Then, I placed a windows 7 disc in the drive and shut the computer off. Without removing the hard drive from the USB port, I turned the machine on and proceeded to reinstall windows 7. I just installed all the updates and I was getting ready to transfer my data when I realized something was wrong.

It is clear that my computer has done some amount of reformatting of my backup drive, and it is being read as an internal drive. My external drive is currently formatted in ExFat. I thought it used to be formatted in NTFS, but I am not positive. When I right click on the drive in Windows Explorer, the context menu is the same as when I right click on my C: drive. There is no option to eject, etc. When I right click > Properties, my 1 Tb drive says "651 GB full, 279 GB free. However, when I highlight all the folders in my drive and right click > properties, it says only "11.4 GB free". Then, when I look in the folders, some of the subfolders still have files in them, but the computer wont let me copy them to the internal drive. Most of the folders that should be full of files are empty and some of the folders are missing all together.

What gives me hope is the fact that the computer says that the drive still has 651 GB full, the amount of data I had backed up originally. However, when I check the C: drive, onto which I just reinstalled windows 7, it says it has 721 GB full, when it should really be very empty because I haven't moved any of my content there yet, and I don't know why that would be, either.

This drive was my backup drive. This means, no, I do not have another backup. I haven't copied anything new onto the drive that could overwrite my original data. Is there an easy way to recover my data in the original organized file structure? Should I leave it to a professional and pay a lot of money that I don't have to send it somewhere to be recovered? How should I even safely disconnect the drive? I was thinking turn it off, disconnect the drive, turn it on, and delete the partition.

 

Read the first two paragraphs over here.
https://www.howtogeek.com/223821/what-is-the-windows.old-folder-and-how-do-you-delete-it/

This means all your data should be in the Windows.old folder.

Backup the Windows.old folder to an external drive formatted with NTFS asap.