You currently have a 500GB SATA SSD
You're moving to a 500GB NVMe SSD.
No problem
You can't move "only the OS".
Rather...you move everything to the new NVMe, wipe the old drive, and then move some things over to the old SSD.
Moving everything is exactly that. Everything on the 850 EVO goes.
Just like this:
Specific steps for a successful clone operation:
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Verify the actual used space on the current drive is significantly below the size of the new SSD
Download and install
Macrium Reflect (or Samsung Data Migration, if a Samsung SSD)
Power off
Disconnect ALL drives except the current C and the new SSD
Power up
Run the Macrium Reflect (or Samsung Data Migration)
Select ALL the partitions on the existing C drive
Click the 'Clone' button
Wait until it is done
When it finishes, power off
Disconnect ALL drives except for the new SSD
This is to allow the system to try to boot from ONLY the SSD
Swap the SATA cables around so that the new drive is connected to the same SATA port as the old drive
Power up, and verify the BIOS boot order
If good, continue the power up
It should boot from the new drive, just like the old drive.
Maybe reboot a time or two, just to make sure.
If it works, and it should, all is good.
Later, reconnect the old drive and wipe as necessary.
Delete the 450MB Recovery Partition, here:
https://social.technet.microsoft.com/Forums/windows/en-US/4f1b84ac-b193-40e3-943a-f45d52e23685/cant-delete-extra-healthy-recovery-partitions-and-healthy-efi-system-partition?forum=w8itproinstall
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Then, you delete ALL partitions on the old SSD.
You can then use it as desired.
Move some/all of your Steam games to it. Designate a folder on the 850 for Steam games.
Then:
Steam games location
In the steam client:
Steam
Settings
Downloads
Steam Library Folders
Add library folder
To move an already installed game
Games library
Right click the game
Properties
Local Files
Move Install Folder