Unable to mount blank DVD-R disc. Location is already mounted error?

The question is a bit unclear about the exact nature of the problem, but I have had similar problems (including the same error message) while trying to burn .iso images to DVD-R. And I, too, was thrown off by track by the error message.

What I thought might work, but doesn't (DVD is grayed out):

• usb-creator-gtk (Startup Disk Creator)
• gnome-disks (Disks)

What unexpectedly did work:

• nautilus (Files) -- Just drag the .iso to the file manager opened to the DVD. When you click the Write to Disc button, it recognizes that the file is an ISO and gives you the option of burning the contents instead of the file.

What also works:

• brasero (Brasero Disc Burner) -- this is what nautilus launches anyway
• cdrecord -v /dev/cdrom filename.iso

By the way, I was able to get rid of the error message "Unable to mount Blank DVD-R Disc" "Location is already mounted" by using dconf-editor to disable auto-mounting as described here: How to disable automount in nautilus's preferences. However, my impression is that the message can simply be ignored. I would consider its presence a bug.

I am running Ubuntu 14.04.

It is a harmless message. Simply close the message window and proceed with whatever you are doing.

CD/DVD writing. I have plenty of "coasters" made by Brasero. Xfburn works great but it won't do some things. I added K3b to my system and it works as it does in KDE. It adds some files which will only be used when you use K3b so they won't negatively affect your system. K3b is the best writing software.

First you should check whether the result is really blank or whether the automounter is clueless. (The statements "blank" and "already mounted" together make few sense to me.)

What do the burn programs report about the medium state ?

Most original info can be expected from backend programs.

From package "dvd+rw-tools":

dvd+rw-mediainfo /dev/sr0

Look for

Disc status:           blank

versus

Disc status:           appendable
Disc status:           complete

From package "xorriso".

xorriso -outdev /dev/sr0 -toc

Look for

Media status : is blank

versus

Media status : is written , ...

If the burn runs with Brasero or xfburn took a plausible time and yielded success reports in the end, then the medium should not be blank. If one of above shell commands afterwards reports the medium as blank, then the burner had no sufficient physical effect. (I.e. demand replacement and hope for a better one.)