Enterprise Vault will mark an index as 'failed' if it had a repeated problem opening/ inserting items. It marks it as failed so that it doesn't repeated log lots of errors when some kind of 'hard' error occurs etc.
So when you do an 'update' it basically says to indexing, set the index status back to normal and carry on updating from where it was upto. So for example say an intermittent error occured then an update would restart the update process and complete. So it could 10 items need to be added, it could be 20,000 all really depends upon when the index got marked as 'failed'
If a hard error occurs (say a physcially corrupt index) then the update obviously won't complete. However because we try and fail again, errors will be logged and so that in itself is useful to help try and determine the root cause.
It is always a good idea to try Update first, as it's quite possible there is no 'hard' issue and it will succeed.
However you should look at errors in the event log to determine why the index got marked as failed in the first place, otherwise the problem may occur in the future again. Perhaps for example Anti-Virus software was scanning the files etc
