Vanilla data folder unpacked is around 9 GB; it does make a difference.
I analyzed a CTD that kept happening in a siege battle in SS campaign.
CTD when I was placing units before battle; I changed a few vidcard settings etc.
"Medieval II has encountered an unspecified error and will now exit"
Then it would CTD when I was placing stakes near gate; ran battle again.
Then battle would start okay but when I went to send a small suicide commando group of 6 mercenary spearman outside the gate to attack the catapult..CTD
Then I started Filemonitor and ran it again.. CTD when I went to move trebuchet closer to wall as enemy catapult approached.
I analyzed my filemonitor log and my HD was frantically looking
in my SS folder for the castle wall texture that has the gate opening ,
right before the CTD.

(I took this screen right before one of the CTDs)
Did it need to reload it into memory for the open/close gate function? It was displaying properly.
Point is:
I checked my SS data/blockset folder to make sure nothing would be overwritten that shouldn't be ( it won't), and COPIED my entire vanilla dat/blockset folder over to SS.
Ran battle again..no CTD; entire siege battle ran fine all the way to finish.