I realize this is a horrible response to this post but I got this EXACT same problem. Eventually I got it to work so if you're as frustrated as I was you'll probably try anything.
- go to setup.slingbox.com
- Click "Get Started"
- Select "Change name or password"
- Change your password (don't ask me why, I just tried this out of desperation and eventually got it)
- From the same menu where you changed your password select "Check for Updates"
And now comes the painful part...
The firmware will try to download, get to 51% and fail, it will prompt you for your password and then ask you to click "Reconnect". Do that.
It will then try to download the firmware again and only get to 25%. It will stall.
At this point I closed my browser (Safari). I reopened Safari and went to setup.slingbox.com again, clicked "Get Started" and "Check for Updates" again.
I went through this process OVER and OVER. Sometimes it would get to 52% before it would crash. Sometimes it would get to 60%. Once it got to 65%.
And then finally after an hour or so of starting and stopping, it got through the entire firmware update.
Sorry that this isn't a magic, definitive answer to your question but as someone who recently banged his head against the wall with this, I thought I would at least post it in case it worked for someone else.
I am having the same problem as both of the above posters and your link does not satisfy the problem. I bought my slingbox pro hd yesterday and have spent roughly 8 hrs attemting to get past the 51% load and stall. Last night, before I ran out of time and patience I got as far as 52% before it stalled and came back with the "firmware error." Today, it went to 25% before it stalled.
Are you using SlingLinks or a very long Ethernet cable? Take a look at this link
I hope this helps