External Storage mismatch

I'm writing up our backup restoration/ procedures and I am not sure exactly how External Storage "matches up" with the corresponding fmp12 database file - that is, whether it is safe to use an older copy of External Storage with the database file, or vice versa.

This is an issue because there are a lot of files within our External Storage (50gb in total), which is Secure. This means lengthy transfer times in the case of a restore (although of course the FMS backup is fast). As such, I've implemented an option within the database to disable the ability to add, edit, or clear existing externally stored container data. This would allow for the users to use nearly all the database's features, and then resume those specific container field write operations once the transfer has complete. Not only that, I've specified that External Storage should be copied over to the drive that the FMS Database Folder resides on, but not the Database Folder, and that once it's been copied over, close the databases, move (so it should be instant) the External Storage into the Database folder, and then open the databases.

The above is based on what could be called pessimistic assumptions about how FileMaker structures, references and maintains External Storage. It could well be that I've made it more time consuming and inconvenient than is necessary.

So I have a few queries:

1. Is it safe to transfer 'on the fly' - so the Secure External Storage files will be transferring over to the Database Folder while the corresponding database file is open and being accessed?
   1. Would I have to disable related container field operations such as add/edit/clear, or even read?
2. Is it safe to 'mismatch' the External Storage and database files? So let's say I restore an earlier backup of the database file, but keep the External Storage as it is, I'm guessing I would lose the container data that was added since the backup (which could be worth the time saved), but would it cause problems going forward?
   1. What about the other way around - restoring an earlier backup of External Storage but keeping the current live version of the database file?
3. If the database file has corrupted, is it likely that the External Storage has corrupted?
4. Is there a difference between Open and Secure external storage with regards to the above scenarios?

Any guidance much appreciated.