Partition on External Hard Drive not showing in Finder
Onyx offers the option of hiding and revealing files, folders and volumes. You'll want to select the Utilities section and then Visibility. This may help with your visibility issue. If you can regain visibility you should be able to select the mounted volume and bring up its info (command + i), then tick the "Ignore ownership on this volume" box.

You could also try logging in as Root and see if that gives you visibility of your missing partition.

Really important bit first, as Root user you have the power to delete files that are hidden/protected under normal circumstances - be careful before deleting or moving anything as you could render your system unusable.

With that out of the way, you enable Root account access by using Directory Utility. This can be found in System/Library/CoreServices or System/Library/CoreServices/Applications, depending on your OS.

First you need to "Click the lock to make changes". Then, from the Edit menu, select "Enable Root User". You will be prompted to create a password for the Root account. Once that's done you can log out of your own account.

Now, log in with the username Root and the password you've just created.

Remember, as Root user you now have the ability to break everything if you're careless - so take your time, do what you need to do and then log out as soon as you're done

You should be able to see straight away whether you have visibility of your errant partition. Hopefully you do. If so, see if you get the option on the info page (command + i) to "Ignore ownership on this volume". If it does, tick it. If not, you could then try hvtuananh's suggestion of changing ownership with the command sudo chown RachelNicholsIII:staff /Volumes/RachelReboot Backup.

If any of that helps then logout and back into your usual account, then disable the Root account using the option in Directory Utility as previously outlined.

Once more, be careful what you do when logged in as Root!