Well, with the caveat that you should only upgrade to fix bugs that you have noticed, or gain features that you definitely want...
(Firmware updates carry a degree of risk)
Samsung.co.uk Support pages don't recognise UA55D6000 or some variants that I spotted while searching.
eg UA55D6000S UA55D6000SKXXY or whatever.
What is your full model number and did you download from the Samsung site or 3rd party site?
However, I did grab a copy for a 6100 and ran it in a folder on my desktop.
The instructions say you did everything right, but the software or instructions may be wrong.
It makes folder structure T-GAS6DEUC > image > files.
Try selecting all the files and copying into T-GAS6DEUC folder and into USB top level.
So you will have 3 copies in total. See if the TV spots what it wants then.
By searching on T-GAS6DEUC, I also found 1 page from someone trying to hack his TV (Root it).
Someone else says UA models should have T-GAS6DAAC
But on genuine Samsung UA6100 download your DEUC seems correct.
I didn't search any further.
Another (remote?) possibility, might be USB flash is the wrong format and, although TV can see files, it's causing a problem.
Reformat USB drive to FAT32 and NTFS or try the TV's Format capability on it if it allows.