Just posting this encase anybody has similar issues.
I fitted a 2.5" internal harddisk to my new solo2 immediately upon receiving it. When I booted it up, it seemed to be working perfectly. However, after about 10 minutes, the remote became very unresponsive, especially when I was trying to open the channel list. Sometimes I'd have to hit the button 10 times before the list came up, other times it would remain unresponsive until I switched off the box. I noticed the longer I left the box switched off, the longer I'd get before it started to be unresponsive again upon startup.
I tried Blackhole, VIX and VTi, with no difference. I dug out an old Harmony 885 remote, set it up and it performed exactly the same. I then removed the harddrive, plugged it into another TV and it seemed to work a lot better. It also worked a lot better with the case off, even with the harddrive in. So I dug out an external usb laptop fan base and sat the box on this, case on with the drive in. I used it for two hours last night and again this morning without any issues.
I can understand a smaller, fanless box like the DM800 having overheating issues with a harddrive plugged in, but the Solo2 has much more space inside and already has it's own fan, albeit a small one. If it wasn't such a phenomenal box in every other way (it laughs at tasks like epg downloading), I'd be disappointed.