I wrote a nice long explanation then lost it so this one is gonna be short
ok,
Auto insert needs to be switched off because of the following.
1) Auto insert periodically checks drives to see if there is any data on them. Now windows isn't interested in blank disks but as soon as data starts being wrote to them it becomes interested. When checking the disks contents (to update explorer) it interrupts the writing streams and usually causes a bad burn.
If using disc at once it messes up early on
If using track at once is will be towards the end and you can usually get away with it by closing the session manually. This doesn't come up with COPY FAILED errors, it just happens in the process of burning.
Now this isn't always an issue (i had it switched on by accident for 3 months and never had a problem on my system but it doesn't usually affect high spec systems. and doesn't always affect everyone so please no blasting about how you can write on your 486 with it enabled. i'm only explaining why companies suggest to turn it off.
Hope this helps,
James