In current cauldron KDE, if brasero is started via right click on an ISO image and the target disk is already mounted and recognized, the brasero burn dialog comes up with the "Burn" button greyed out, and then covers itself with a popup saying "Please wait until the estimation of size is completed". This popup stays there until cancelled. If you eject the disk and start brasero in the same way, the burn dialog comes up requesting a disk. If you load the same disk, as soon as it automounts, the burn dialog changes with the "Burn" button enabled and you can proceeed without any problem.
this seems more an upstream bug to me ( I fear seeing their lists at bugzilla.gnome :s)
Keywords: (none) => UPSTREAMURL: (none) => https://bugzilla.gnome.org/show_bug.cgi?id=681440
This happened consistently for months, but is no longer happening in cauldron.
Status: NEW => RESOLVEDResolution: (none) => FIXED