Header synchronization

So the DB files are the full contents of your Archive split in to quarters of the year based on sent/received date
So you have like

2014_01_03_001.db -- this contains all email for January to March 2014
2014_04_06_002.db -- this contains all email for April to June 2014
2014_07_09_003.db -- this contains all email for July to September 2014
2014_10_12_004.db -- this contains all email for October to December 2014

So if you've had emails dating back to 2004 you'd theoretically have 40 DB files.

The MDC is the MetaDataCache, which is used for the Header Synchronization.
This contains the full folder structure of your archive and brief snippets of each email.
The data is things like Subject, Sent Date, To, From, Size etc

This MDC is whats mounted in Outlook and displays as your Virtual Vault.
When you click an item, it then looks in an XML file to determine which of the DB files the item exists in and pulls the item from the DB file.

You can have MDC without the DB files, which is contentless cache.
When this happens and you click an item, it gives you a yellow banner and the first 200 chars or so of the email so you can hopefully determine if its the right email, to open the full email you have to double click it and it will retrieve the item from the server

There used to be a policy in old versions of EV9 and below where you could have it retrieve an item from the server to show in the preview pane, but its a performance killer, so if you have someone who is using the down arrow to scroll through email, you could potentially be making 100's of calls to download.asp which isn't particularly good if you have lots of userrs doing it.

You can also have the DB files without the MDC, which is just regular Vault Cache.
Meaning that if you have shortcuts in your mailbox to archived items, when you double click it, it will retrieve the item locally and not from the server, and if you're offline (or depending on the setting) it will use "Offline Archive Explorer" -- so you can browse your vault cache and open email from there if you don't have shortcuts.

Typically the MDC can be anywhere between 8% - 15% of the archive.
So if you have a 5GB MDC, then it could be safe to assume that the entire archive size is somewhere around 50GB in size, though you can control the size of the vault cache through the policy where it will only keep the newer items on disk and remove the older items.

MDC files cannot be controlled in terms of size though, it will grow as big as it needs to.
And when you drag and drop items from your mailbox to virtual vault, the full items are placed in to the MDC, then when you synchronize, its placed in one of the DB files and made in to the snippet like all the other items.

The MDC does not shrink though, so if you have say a 120MB MDC, and then you drag and drop in 1GB of data, you then end up with a 1.1GB MDC, you sync and if though it may now make the MDC 180MB, the file is not compressed so will still consume 5GB

There are certain plugins that can intefere with how Vault Cache operates. such as the Voltage 6.x client, Windows Desktop Search and Antivirus, especially with on demand scanning.

As for 0x800408013 -- that's a difficult one, typically its an error you sometimes see with Outlook, but generic troubleshooting advice is given but no real concrete explanation for what it is.

I did however see some saying that the PST was corrupt.
So what you could do is close Outlook, go to your Office15 directory and run ScanPST against the MDC file and try again, because your folders probably aren't showing because it can't create them.

One other thing you could do is just rename the MDC file and resync and it will recreate the MDC from scratch, but if you truly have that large of an archive, could take some time to rebuld