It can be seen on the attached screenshot, that K3b does not burn a 23.1 GiB .iso on a 23.3 GiB medium (on Kubuntu 16.04 LTS). This also happens on a current Manjaro Linux (more details will follow). (The .iso was first created with backup data with K3b by selecting to create a local .iso file without burning in the burn dialog). The burner is a Pioneer BDR-XD05T external USB.
Created attachment 109177 [details] Screenshot showing the problem
On Manjaro Linux with K3b Version 17.08.3 the console output when the start burning was clicked is thus: K3b::ImageWritingDialog(0x7ffddc9c34c0) QLayout: Attempting to add QLayout "" to QFrame "", which already has a layout kernel version: "4.9.63-1-MANJARO" connecting kernel version: "4.9.63-1-MANJARO" "Daten werden vorbereitet" "Warten auf ein Medium" QLayout: Attempting to add QLayout "" to K3b::EmptyDiscWaiter "", which already has a layout QLayout: Attempting to add QLayout "" to K3b::EmptyDiscWaiter "", which already has a layout Waiting for medium "Leeres Medium" "DVD-R, DVD-R sequenziell, Zweischichtige DVD-R, Zweischichtige DVD- R sequenziell, Zweischicht-DVD-R-Sprung, DVD-RW, DVD-RW Eingeschränktes Überbrennen, DVD-RW sequenziell , DVD+RW, DVD+R, Zweischichtige DVD+RW, Zweischichtige DVD+R" "" "/dev/sr1" ------ nothing useful found. it seems the last part starting with "Waiting for medium" is the interesting part: it lists different DVDs but not the BD which is already inserted.
*** This bug has been marked as a duplicate of bug 381074 ***
Thank you!