I am looking to buy an external 1TB hard drive to store sensitive information on. External hard drives aren't (physically) small, and it may have to be left unattended at times.
What can I do to increase the hard drive security so that if any physical or software tampering is done, I would be able to detect it?
To protect against physical removal and later unauthorized access, just encrypt it completely using one of the available software like VeraCrypt (container-based or full encryption). In such a scenario getting it will make it not accessible anywhere.
On the software part, you will have to secure from unauthorized access the OS used to access the drive. If you have encrypted the drive and you must enter a password for it upon restarting the OS and you secure the OS against possible unlock scenarios you should be fine.
When it comes to a preventative countermeasure for physical tampering, you can use a rosin like Epoxy while the Hard Drives and ATA/FATA cables stay connected with each other, and she stay in her enclosure or naked.
Secondly, If you want to prevent that files get tampered take a shot of your drive using a tool (e.g dcfldd) just to make sure you've the original in your hand, further encrypt the disk the cold way as said in a comment.
