Well for starters go to this post https://community.slingbox.com/message/46379#46379 and look at the comments from;
SlingJames Jul 22, 2011 4:38 PM
Ghostman Jul 27, 2011 4:35 PM
BurnsNH Aug 14, 2011 1:05 PM
jeff74 Aug 30, 2011 7:34 PM
jhenrichs Sep 6, 2011 8:22 PM
ManxMariner Oct 26, 2011 4:29 PM
ManxMariner Oct 26, 2011 4:33 PM
robbosling Dec 22, 2011 10:00 PM
captricky Dec 31, 2011 8:46 AM
Bugoy19 Feb 7, 2012 8:17 PM
nacca69415 Feb 8, 2012 9:32 AM
OK, checked that and it's not related to your problem
1. That was a Slingbox Pro NOT a Pro-HD. It seems to be the Pro-HD that is ultra-senstive to power supplies.
2. As far as I can tell none of them mentioned that the horseshoe light was permanently on ?
3. Apart from the OP no-one mentioned that the reset wouldn't work ?
So totally different problem. As I said, yours are classic symptoms.
I have a Slingbox Pro HD and it recently stopped working. The front panel shows the upside-down "U" lit up, the power light lit up and the network light lit up but, it is very dim. I have tried pressing the reset button but nothing happens. I have tried pressing and holding the reset button for up to 60 seconds, but nothing happens. I have tried unplugging the device for about 12 hours and plugging it back in but, it just goes immediately back to the panel description above. I fear the unit may just be dead. Anyone have further suggestions or information? Thanks.