There don't appear to be a large number of viable solutions available for secure hardware-encrypted hard external drives. I used to recommend the Maxtor BlackArmor for this type of application, but those are no longer available. The Seagate BlackArmor drives are NOT hardware-encrypted - so don't be fooled by the continued and confusing reuse of the BlackArmor name.
The best choice on the market right now appears to be the Apricorn Aegis Padlock drives. These drives offer features and security comparable to the Ironkey or Kanguru Defender/Elite USB thumbdrives, but in an external USB drive form factor.
1). Works with both Windows and Apple Mac (including Time Machine - you have to reformat drive for Mac use)
2). No special software required on client endpoint for either Windows or Mac
3). Has support for IT admin passkey and several user passkeys. So an IT admin can recover data in case the user forgets their password or leaves the company.
4). No admin rights required on Windows for normal use, but is needed for initial setup
1). Not currently FIPS certified from what I can tell
2). 6-16 character password means keyspace is a a little small. This is mitigated by the brute force protections the drive has.
3). Only has only USB connector and the cable is short. Drives does come with longer Y-cable in the box.
4). No enterprise management console is available if you are looking to manage and support a fleet of these.
Amazon has good prices on these. Links (affiliate tagged) to the 128- and 256-bit versions of the three largest capacity drives available right now are below:
email: david @ sharpesecurity.com