In the interests of clarity, I shall refer to the two computers herein as Tux 
and Dozy (their names).
Over the weekend I edited the /etc/fstab file on both computers.  I added a 
second hard drive to the /etc/fstab on Tux, with a view to using cron and 
rsync to back up for me.  The only point of this example is that it worked, 
without a hitch, and has continued to work.  (I.e. 2 hard drives are mounted 
when the systme boots up.)  So I must have been doing the right thing in this 
case at least.
Dozy was a different story.  It is quite a long saga and I don't know how much 
detail you need.  So I'll give a summary and perhaps you would be kind enough 
to ask questions if I have given you the wrong bit or not enough info.
The aim: mount one of 2 DVD ROM drives at boot up.
*I duly edited /etc/fstab, changing the following line
/dev/cdrom1    /media/cdrom          udf,iso9660 defaults,user,noauto,ro   0      
0
to
/dev/cdrom1    /media/cdrom          udf,iso9660 defaults,users,auto,ro   0      
0
I thne rebooted and all was well - the DVD ROM drive was mounted at boot up.
I rebooted again - and the DVD ROM drive was unmounted and the fstab back to 
what it had been in the first place.*
Repeat from *to* until thoroughly fed up (in my case some 6 or 7 times).
I decided that it must be using something somewhere on the computer that 
contained the original file.  Perhaps it was using the back-up I had created 
of fstab before messing around with it.  So I duly edited the back-up file, 
the back-up of the backup file (created by the computer) the backup of fstab 
created by the computer, and rebooted.  
Again, fine the first time, a failure the second.  So I delved.  fstab was 
back to its original form, and another backup file created by the computer in 
the original form.  So, I edited both files that now held 
"...user,noauto,ro...".  Again, fine the first time I booted up, back to 
square 1 the second.  So I tried deleting the references to any kind of user 
or any kind of anything other than defaults in all the files.  Same result.  
By now I have got fstab, fstab~, fstab~~, fstab~~~, fstab~~~~, fstab~~~~~ and 
fstab~~~~~~.
So I changed tack.  I deleted all the files, including my own back-up, except 
fstab itself, which I reedited.
That appeared to have worked.  It survived 4 or 5 reboots - but has now gone 
back to what it was in the first place.
I am at a total loss.  Both systems are running the same distro (Libranet 3 
updated and upgraded with Sarge).  I had no trouble on Tux.  Why is Dozy 
being so troublesome?  And what do I do to solve the problem?
TIA
Lisi