I know I can make a new container on a new drive. Here is my situation:

My Firefox's directories are stored on two virtual TrueCrypt drives:

1. the standard Mozilla directory with the extensions, cache, bookmarks, etc. on one 2 GB Z: drive. Needs to be 2 GB as that is the requirement of backing it up in encrypted form offsite.

2. The Firefox downloads directory which can become very big is on a 100 GB TrueCrypt Y: Drive.

When I tried to setup a sandboxed Firefox in my regular C container, the contents are displayed in their respective sandboxed Y: and Z: drives, which isn't good since they are permanently unencrypted.

I can't move a single container to the encrypted Z: drive as it isn't big enough for the Y: downloads directory. Nor can I place it in the Y drive as the Mozilla directory must reside on a 2GB drive.

Question: Is there a way to have the sandbox container reside on Z while downloading either to the Y: download directory proper or to a sandboxed Y directory on the physical disk Y:. I.e., can the container be split over two physical drives or can the downloads from the Z sandbox feed the Firefox loads directly to the non-sandboxed Z download directory.

EDIT: What would do the trick:

Sandboxie Control > Sandbox Settings > Applications > Web Browser > Firefox

as seen here:

http://www.sandboxie.com/index.php?FirefoxTips

added this:

[] Allow direct access to Firefox downloads folder.

Then I could keep the sandbox container for Firefox on the Z: drive and all downloads would go to the non-sandboxed Y:\Firefox downloads\ directory.

Thoughts?

I'm afraid that I don't follow what you want to do.
Maybe this will help ...

Do sandboxed and unsandboxed Firefox use the folders/files on Y: and Z: whenever they run?
If so, then those TrueCrypt drives are mounted before Firefox runs.

Sandboxed Firefox can read files from those drives when it runs, but cannot write directly to those drives unless you allow it.
It will normally write to the corresponding folders inside the sandbox:
C:\Sandbox\(user)\(sandbox)\drive\y\...
C:\Sandbox\(user)\(sandbox)\drive\z\...

If you want Firefox to write to the actual folders on Y: and Z: you must set those folders as Direct Access folders for firefox.exe.
Then all writes to the folders that you set as direct access will occur outside of the sandbox.

For all downloads to be written to Y:, you must use a Direct Access setting for sandboxed firefox.exe.
Sandbox Settings > Resource Access > File Access > Direct Access
"Add Program: button: firefox.exe
"Add" button: Navigate to and select the downloads folder on Y: drive
OK

That's the equivalent of your:
[] Allow direct access to Firefox downloads folder

To allow sandboxed Firefox to read/write directly to it's profile files on Z: drive, you need to do 2 things:
Check mark the item:
Sandboxie Control > Sandbox Settings > Applications > Web Browser > Firefox
"[+] Allow access to entire Firefox profile folder"
Click "Apply"

then after that:
Applications > Folders
(with firefox.exe showing in the box "Select folder for")
Click "Add" and select the actual Firefox profile folder on Z: drive.
OK

Thank you very much. That is exactly what I wanted to do.