Well it seems to have worked OK for me as well. I bought a 2 Tb Western Digital Elements Hard Drive for £87 off Amazon. I connected it to my laptop and used the free Easus software to split it into two EXT 3 partitions. It took somewhere between 5-13 hours ( I left it running overnight)and I ended up with 2 partitions of about 930Gb each. I connected the drive to the USB port of the Humax 500TB Foxsat and it picked up the 2 new partitions. For some reason the Humax addressed them as SDA 5 and SDA 6. I created a couple of files on both the partitions using File Manager and then copied some programs from the internal drive to SDA 5 and also to SDA 6. They copied OK and afterwards the Media Guide was showing the correct utilisation of space on both partitions for the sizes of files I had copied. I found it took about 15 mins to copy a 1 hour HD programme. Last night we watched programmes from both partitions ( and then deleted them) in exactly the same way as we had been doing from the internal drive - so unqualified success.
It looks like I have upgraded my Humax from 500 GB to 2500 GB for £87 - which I think is excellent news. Let's hope the Western Digital Hard Drive is robust enough.
By the way I can't see any way in which I can rename the SDA5 and SDA 6 partions into something more meaningful - has anyone got any ideas?
Many thanks to Graham Thompson and the other contributors to the Forum who pointed me in the right direction. Hope this post helps others.