This data is for debian 9.3. However, I remember seeing the problem
with other debian versions and with other hardware systems.
I copy each dvd of the debian software repository from physical dvd media to iso file in a previously created path. I run one line of code at a time, and insert corresponding dvd each time.$I check the iso files with MD5SUMS. The first and last iso files are always good. However, the other iso files are usuially corrupt.
To make all the iso files good, I restart the system after each run.
Note that the iso files are all about the same size, except
the last one, which is smaller.

when system starts up, message says watchdog did not stop!
Here is some information I found about watchdog.
systemd sets a "hardware watchdog" timer as a failsafe, to ensure that if the normal shutdown process freezes/fails that the computer will still shutdown after the specified period of time. This time period is defined in the variable ShutdownWatchdogSec= in the file /etc/systemd/system.conf.
#RuntimeWatchdogSec=0
#ShutdownWatchdogSec=10min

So, how can I create the iso files, without restarting the system after each one? And what is causing this to happen?
For more details, see pdscript.com/d9316.txt.

Probably the dvd was auto-mounted. Probably you can find some related messages in /var/log.

I would say you are pulling the disk out before it's finished writing. Add " && sync" (leading blank) after each command, theat way the prompt won't return until the write is complete.

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Does auto-mount have anything to do with this? Should I change a setting? I found many messages in /var/log. Can you be more specific? And after each run of dd, I carefully checked the terminal for the ending prompt stretch@debian:~$. But you say that does not necessarily mean the write is complete? And there were no error messages in terminal. I think it is unlikely that in 12 out of 14 runs, I pulled the disk out before it's finished writing. But I admit it might be possible. That sounds like a good idea, Add " && sync" (leading blank) after each command. I might try it next time. Any more ideas? Anyone willing to test? Most of the dvds I used are 4.7gb. I suspect any dvd will work as a test, if you have the MD5SUM for it.

I'm really sorry, we cannot test it for you. It is exactly you who made a mistake, and you need to find it.
Yes, if you mount a drive (dvd, usb, whatever), some of the filesystem info will be kept in memory. But probably not related to your problem.
Yes, probably dd writes into a cache and you will get the prompt back before the write really completed. That is normal in linux.
you need to check messages related to the dvd device, you can use grep dev.sr0 to find them (for example).
If md5sum did not match the content of the dvd is corrupted. Either because of it was not completely written or because of some "other" things was "remembered" and saved...

Next time I do this, maybe I will try something different.

The dd command normally does a sync() before exiting. You can see this when zeroing a slow device like a USB flash drive. You'll see the "no space on device" message from dd, and then a delay (sometimes several minutes) before the next command prompt. The delay depends on the speed of the device and how many gigabytes of buffer remain to be flushed out.