Startup Disk Creator myster(ies)

Recalling my unresolved attempts to create a Trisquel Live DVD+R with data files, I bit the bullet and bought some DVD+R DL disks. My first attempt, in which I simply selected a source iso image of Trisquel 7 that I had previously downloaded and the DVD+R DL disk as the target, instantly produced a "failed" result when I tried that. Dmesg reveals nothing.

Now I'm trying again. Noting that the DVD+R DL disk is identified as "HL-DT-ST DVDRAM GT50N, that it is located at /dev/sr0, and that it has a capacity of 2.0 kB, I chose the "erase" function, which seems to be doing something very slowly and with 9 percent of RAM in use. Is the system actually "erasing" a brand-new DVD+R DL disk ? If so, how long will that take on the 8.5 GB capacity DVD+R DL disk ?

I suspect that the DVD+R DL disk is simply a double-layer DVD and that it is _not_ rewritable ... But I could not find any other choices than CDROM, DVD+R and DVD+R DL at my local Micro Center store.