Upgrade to 3TB possible?

The practical experience so far says 2TB is the limit on a USB drive, and don't ask the Humax to format anything over 1TB.

I don't know what would happen if you externally formatted a larger drive and installed it in the Humax internally, but aren't they rather expensive?
I don't think the kernel supports GPT partition tables. However, it may be possible to use a standard MBR partition table with a sector size greater than 512 bytes.

It might be best to wait until someone reports success with an external 3TB USB drive before attempting to replace the internal drive.
As stated the Formatting limit using the Humax is 1TB so you would need to format the drive externally before connecting it to the Humax, using a P.C. If the Humax has the Custom Firmware installed it is possible to format a 2TB drive by following the link HERE, However no-one has tried 3TB as far as I am aware