I think that the only problem with external HDDs may be power related, i.e. TV's USB port may not deliver enough power to HDD. However, this should not be an issue with any modern HDD (2.5", USB 3.0). USB 2.0 should not be a bottleneck. Cable TV channels usually have bitrate of up to 10 Mb/s. That is 1.25 MB/s, which is supported by any HDD and is way under the capacity of USB 2.0.
Here is the list of drives that my TV (49UH8507) was able to use as a recording device:
1. WD My Passport Ultra 1TB, USB 3.0 plugged into USB 3.0 port on TV
2. Apacer AC233 1TB, USB 3.0 plugged into USB 3.0 port on TV. It also works plugged into 2.0 port.
3. An old Apacer AC203 500GB, USB 2.0 plugged into USB 3.0 port on TV. This HDD was designed to draw power from two USB ports using Y cable. However, it successfully works with a regular mini USB cable.
4. Sandisk Extreme 32GB, USB 3.0 flash disk
5. An old Silicon Power 8GB 3.0 flash disk, plugged into 2.0 port on TV. I successfully recorded some cable TV HD channel .
Virtually anything I tested was working ok.