How do I know, how many writing cycles on a RW disc sector have been completed and how many are still remaining?
For DVDs, you can read the Recording Management Area (-RW) or the Disc Control Blocks (+RW) using the READ DISC STRUCTURE SCSI command (see SCSI or Mt. Fuji). The descriptors keep a running modification count as well as other interesting information, like the model and serial number of the drive that recorded it!
I see. But it needs to be considered, that the inner circles are written onto more frequently.
UDF sparse/sparing tables can keep track and balance the outer/inner edge writing cycles, but once the disc is reformatted, the sparsing tables, which is a part of UDF itself, is lost, as far as I know.
Another approximate way is verifying the burning quality. It should not decrease much for the first 900 cycles, but then, it should plunge.
One of the forum members documented the degradation of DVD-RW after 40 burns:
Is that degradation also visible on other (especially old) drives? (A Pioneer BD-RW should be reliable)
What about 150 rewrites?
How high is the writing speed?
With 4x all the time
I noticed that when I rewrite RWs 5 times a day, I have much less degradation than when writing 5 times over a whole month.
What if you wait one month and then write 5× per day? Still degrading more?
The first write after one month shows much degradation. Some more writes at the same day only a little.
Is it possible, that the disc degraded itself over the month?