Erasable (RW) blanks are fundamentally different from write-once media. They may look similar, but that is misleading.
A write-once blank (CD-R) uses a dye which changes composition when exposed to strong light of the right frequency. That change is not reversible; the new form is stable and absorbs the reading laser differently from the original. An erasable disc (CD-RW) uses an alloy instead of a dye. When exposed to light of the right properties, it changes from crystalline to amorphous; another form of exposure switches it back to crystalline. The reflectance of the alloy depends on its form. Essentially, as a crystal it reflects the read laser as though it were a dull mirror; in amorphous form, it scatters the illumination. Instead of getting white and black (like a pressed disc) or near-white and near-black (like a write-once), an erasable provides light and dark gray and needs more sensitivity in reading.
As a result, only drives marked MultiRead are able to read your erasable reliably. Very few audio CD players will read RW discs. Otherwise, you can use mastering software on an erasable exactly as you would on write-once and expect the same results. That makes erasable media valuable for a couple of reasons. If you're trying something you have not done before, your failures will not fill the trash with non-recyclable plastic. More to the point, the variation in erasable blanks is much less than that in write-once. Therefore, if you cannot tell whether a failure is due to your system or your medium, try an erasable.
UDF is implemented quite differently on erasable and write-once media. For details on that, please see the pages on packet writing. In summary, a CD-RW written in fixed-length packets cannot be finalized to ISO 9660 Level 3, but it can be read in a MultiRead drive with an appropriate driver installed.
When an area of the disc is erased, it is converted to be all crystalline - or at least, as nearly so as possible. In practice, there is always some amorphous material (as there is some crystalline material left when written to be amorphous). With each write/erase cycle, a bit or a region of bits becomes more nearly balanced between amorphous and crystalline. As a result, after a number of cycles, the two states become less distinct to the reader and the ones and zeroes approach 'halves'. Nominally, a disc will survive 1000 such cycles, but individual regions may fail sooner. During formatting for fixed-length packets, the software can determine that a region is failing and mark it to be bypassed when writing. However, there are some critical regions - such as where the TOC begins - which cannot be bypassed. Without packet writing, bypassing is not possible.
When any disc is to be recognized by a drive, either by being inserted with AIN on or by being selected by software, the drive attempts to find out what sort of beasty it's dealing with. To do that, it must find the start of the TOC and read data from it. If it gets nothing at all, then a writer assumes that it has a blank; a reader keeps trying indefinitely. Let's assume that we have inserted an erasable with a bad TOC into the drive so that we can erase it. If the TOC is bad enough, even a writer will not be able to make sense of it and will keep reading indefinitely. It gets enough to know that the disc is not blank, but not enough to figure out what it has. That disc defies erasing because erasure cannot start until the TOC is read.
If the TOC can be read well enough to let the drive stop, but not so well that it can be deciphered, you have an 'Unrecognizable Format'. It needs to be recognized so that a quick erase is possible - one which clears the TOC to allow writing without clearing the whole disc. You may not intend to use quick erase, but the software is not psychic, so it will not let you start doing anything until it knows how to do what you might intend. It could accept the condition and simply gray out the quick option, but I have found no program which does that. Fortunately, that problem disc may be erasable with a program such as Super Blank (linked from this site) which offers no option but always does a full erase.
Is all hope lost for the disc which will not stabilize? Perhaps not. There are three options, none of which is certain but each of which has been reported to succeed occasionally.
When (if?) quantitative data are accumulated, I'll be happy to post them. When erasable blanks cost $5-10 each, recovering them was worth the effort and I had some success with the first two methods. Now that $2 will buy an erasable blank, I'm more likely to discard a flaky one.
E-mail me at cdrecording@mrichter.com
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