How do I unlock a 2TB USB drive on macOS High Sierra?

Based on what you posted, it appears you're running into a permissions issue on your 2TB USB drive. If the drive mounts and you can open it but not write to it, it's likely either locked via permissions or there's an issue with its format or disk flags.

Let's start by verifying how this drive is formatted: either as Mac OS Extended (Journaled) or APFS. To do so:

1. Open Disk Utility
2. Select the drive from the sidebar and look under the Format field.

Next, let's double-check the drive's permissions.

1. Right-click the mounted USB drive on the desktop or in Finder and choose Get Info.
2. At the bottom of the window, check the Sharing & Permissions section.
3. Make sure your user account (or “everyone”) has “Read & Write” access.
4. If it shows as locked or permissions are wrong, click the lock icon (bottom right), authenticate, and adjust the permissions.
5. While still in the Get Info window, check the box for “Ignore ownership on this volume”.

• If it's not checked, and you can’t change it, run the following command in Terminal: (Note: Replace YourDriveName with the actual name of your USB volume.)

sudo diskutil disableOwnership /Volumes/YourDriveName