No matter what, remember that 4.7GB is sleazy marketing GBs, i.e. 1000 and not 1024. If translated to actual capacities, single layer DVD±R[W] capacity is only 4.4GiB, and 26GB BluRay Disc is actually 23.3GiB. Keep this in mind when creating your ISO images to burn manually.
I purchased a Blue-Ray writer (an LG-brand Hitachi Model WP40NB30) hoping to use the 25GB M-Discs to archive some of the files I have accumulated spread out over several computers.
Fortunately, the growisofs tool comes to the rescue, so from the command line, I just execute as a regular user:
$ growisofs -speed=1 -Z /dev/sr1=big-image.iso
I need the lower speed to keep from prematurely emptying the buffer which borks the disc.
Although older, this page provides plenty of background on what is happening.
K3B does not support Blu-Ray burning out-of-the-box, but it can be configured to do so. still, it will not automatically break up files that are too large to fit on one CD/DVD/Blu-Ray medium.
Some discussion of bootable CDs, bootable USBs and partition offsets (very interesting) cane be found here.
Burn, baby, burn.
I have had success using Brasero as a graphical tool for creating 20+GB .iso images. It's handy and tells me how large an image has actually been created so I can choose which medium to use most efficiently, but Brasero will not burn a Blu-Ray disc. I am, however, able to do than manually by:
$ xorriso -as cdrecord -v dev=/dev/sr1 blank=as_needed image.iso
After all this, successful burning of Blu-Ray discs was not repeatable. The only way I have found to successfully burn these discs is to run the win-32 application ImageBurn. Installing it in Codeweavers Crossover was straightforward as the installer was automatically downloaded and the bottle created, but I encountered two problems.
The first was that I was unable to create a link to /dev/sr1 that shows up in the Wine Configuration/Drives window; the option to do that (the Device: selection) was grayed out, likely because, as a regular user, I needed superuser permission to do that. Crossover states in section 9.3 of their User Guide that invoking:
$ ln -s /dev/sda1 "~/.cxoffice/bottle-name/dosdevices/j::"
That should create the drive, but I was unsuccessful using that. Perhaps being the superuser would have made that possible. I finally added drives by creating one using the Wine Configuration/Drives window, the editing the symlink itself in the dosdevices sub-directory of the appropriate bottle to point to /dev/sr1, my Blu-Ray burner.
That problem solved, I turned to my second problem. ImgBurn was not able to see or connect to the drive. Under Tools/Settings/"I/O"/Page 1, I checked "SPTI-Microsoft" instead of the default "ASPI-WINASPI32.DLL" and the drive immediately became available to the ImgBurn.
Finally, everything works as advertised. My discs are recognized properly and burned properly. I am able to use ImgBurn's tools to select directories and burn directly to both the 25GB M-disc media and the double-layer DVDs without issue. I have not yet tried spanning several discs.
The sad thing is that interacting with Blu-Ray media is certainly possible, creating iso9660 archives that span multiple discs is possible, so why haven't those capabilities been incorporated in Linux applications?
Blu-Ray Linux Wiki
Welcome to libburnia project
The state of Blu-Ray burning in Linux is terrible
XORRISO man page examples
Ray's Notebook § Computing