Multiple sessions on cd/dvd

[nsa]

I just installed Windows 10 under VirtualBox (version 4.3.12 r93733)
I am using a CD-RW to transfer files between my host (Windows 7) and VB.
I found that I had missed some files, so I added them to the disk.
VB cannot see the added files.
I have checked this with an instance of Ubuntu 14.04 and the additional files remain invisible.
I cannot find anything in the documentation about this.
Have I missed something?