Appending to non re-writable DVD
Can you still burn/write additional files to a used non re-writable DVD?
For example, after I burn a 900 MB .iso to a DVD, can I still write additional files to that same DVD in the future?
It all depends on how you burned the CD or DVD.
Some software "finalizes" the DVD preventing more data from being written, others create a file system on the disk that has room to grow so files can be added and "deleted" in the future (if the disk is not a read/write disk and you delete the file, you would only be hiding it from the file system, you would not get the free space back and recovery software could still read it).
Typically when you burn an ISO image file most software I have seen will finalize the DVD at the time of the burning, not allowing any more files to be added. If you want extra files added you will need to add them to the ISO image via an ISO image editor before you burn the DVD.
Yes, you can add files in the future. That's called "multi-session". What this means is that the next time you want to add a file your burning software builds an ISO filesystem containing this new file(s) and, in addition, it contains "links" to the previous files already existing on the CD (or DVD). This new data that you burn is called a "session". When you mount this CD, the computer searches for the last session and mounts it.
But you can only add a session if the last session isn't "closed". So you won't be able to use this technique to salvage empty space on your old CDs.
Another issue to consider is where you want to use your CDs. Some old devices (e.g., old MP4 players) don't support multi-session: they show you only the first session on the CD.
Yes and no. Physically, yes, you can update the DVD with additional data unless it is finalized. However, as you have burned ISO9660 filesystem, which has fixed size and content, that won't do you any good as you would not be able too see it without extra effort (you could, for example, probably use 'session=xx' parameter to mount(8) on GNU/Linux to access it)
If you want to append data to your DVD, you should use UDF instead of ISO.