CD Recording
"CR-RWs need to be Formatted"
"No They Don't!"
How many times have I seen a plea for help along the lines of "I've just formatted a CD-RW disk and now I can't write to it, the software tells me it's full"? Too many!
It seems to be one of the first things to confuse new users of CD-R technology. You can format a CD-RW if you want to, but it depends on how you are going to use it. In other words, you don't format it because it's a ReWritable (as opposed to Recordable) disk, you format it if you want to use it in a certain way.
Recording methods
Mastering
The "standard" way of recording CDs is called mastering (sometimes called pre-mastering, but it means the same). This is the way all manufactured CDs are organised, and requires the entire CD's contents to be assembled, and then written to the CD in one session. When you use a program such as EZ CD Creator, WinOnCD, Nero, CD Maker, etc. you are mastering a CD.
You normally start with a blank recordable disk, then using your mastering program you pick all the files and directories you want to put on the disk, then you burn it. You can burn to a blank CD-R disk, or you can equally well burn to a blank CD-RW disk, at this stage there is no real difference. Notice the word blank. Blank means just that - blank, empty, nothing, nil. If you write any sort of format on the disk first then it isn't blank!
So a CD-RW disk can be used just like a CD-R to write a mastered disk. You don't do anything to it before you start using it.
Multi-session
This is a way of using the mastering method to append data to a disk. If the first session you write doesn't fill the disk, and you don't close the disk, you can later add another session. In the new session you can add more files, "replace" existing files, or "delete" files. However, the new session can only add to the disk, it can't alter anything in the previous session, so when a file is "replaced" a new copy is added to the disk and a new directory entry supercedes the old one. "Deleting" a file just omits the reference to the file from the new directory.
You can keep adding sessions, increasing the amount of data on the disk until it's full. It works the same with both CD-R and CD-RW disks.
However, when a CD-R is full it's full, there's nothing more you can do apart from read it. A CD-RW can be re-used, but only by completely erasing it and starting again. In fact a "complete" erase can be done quickly because all that is really erased is the table of contents (TOC) at the beginning of the disk. It then appears to be empty and a new session will just overwrite anything that's actually on the disk.
Packet writing
Writing a CD in sessions creates a highly standardised and hence portable disk, but isn't always very convenient. It would be much nicer to be able to use a CD like a diskette, and since CD-RWs are re-writable just like magnetic disks, why not?
Why not indeed, and you can do this using a technique called packet writing. This turns a CD-RW into a true random-access disk, and like all random-access disks it works by dividing the disk area into blocks, so that each block can be independently written to and read from. Before a disk can be used in this way a pattern of empty blocks has to be recorded on it, a process called formatting. You do exactly the same thing when you use a diskette, although they are often supplied ready-formatted in the box.
Packet-writing also requires a driver to be installed into the operating system, because you no longer need special software to write to a CD. Any program that can write to any disk drive can write to the CD, it just appears to be another disk. However, you can't necessarily read a packet-written disk on any computer, even if the CD drive can physically recognise CD-RWs. The data layout is different from a mastered disk, and a computer needs to be able to read UDF format to recognise it. The latest versions of Windows can read (but not write) UDF, but even so there are historic compatiblity problems between different packet-writing products. So packet-writing, while convenient, is far less portable than mastering.
Bottom line
CD-RWs lend themselves to packet-writing, especially for applications like keeping backup copies of data where they only need to be read back on the same machine. To be used for packet-writing, CD-RWs must be formatted, but that is not the only way to use them. They can equally well be used for mastering, and then bulk-erased as required for re-use. For that purpose they must not be formatted.
Formatting also explains why a packet-written CD-RW only holds about 550 MB of data. CD-RWs aren't lower capacity than CD-Rs, they still hold 650 MB when mastered (assuming standard 74-min disks). As with magnetic disks, the formatting information occupies some of the physical space on the disk.
- Note
- To convert a formatted CD-RW back to mastering use, just erase it the same as erasing a full mastered disk.
Packet-writing CD-Rs
CD-Rs can also be packet-written, but I've avoided the topic to prevent further confusion! It's also a useful technique, but works rather differently from packet-writing CD-RWs since space on the CD can never be re-used. As this article is about formatting CD-RWs I won't go into it further (there are plenty of other references) but just mention the subject for completeness.
Site sections:
