For anyone interested, this is how to have a fully encrypted USB portable dropbox with full syncing. (windows only unfortunately.)
disclaimer: None of these applications are mine. Be informed and read the ToS for everything. Everything is done at your own risk and the risk of your data. Backups are your friend.
-Dropbox portable (http://nionsoftware.com/dbpahk/download)
-Default "home" pc with Dropbox and Boxcryptor deployed, and data synced and encrypted.
-local admin rights on the relevant other computer
-Large enough USB for what you need.
(The USB I use is encrypted with Bitlocker, and I have a Veracrypt container on it for other data. Everything works with no issue.)
- Install DropboxportableAHK on the USB, it will prompt for log in details and directories.
The installer is straightforward. Follow instructions as detailed.
- Allow Dropbox portable to sync.
- Install full Boxcryptor application on the PC (not Boxcryptor portable.)
- Sign into Boxcryptor normally.
Once Boxcryptor loads, you can then browse to the dropbox folder location in the USB via boxcryptor to access the encrypted folder.
Boxcryptor then allows you to work with your data normally, as you would if you were working on a locally installed version of Dropbox, and syncs the data on the USB as well.
Shutdown Boxcryptor and Dropboxportable and eject the USB before unplugging it.