I've just moved my client over to a hosted exchange mail box....
He uses some open source software (i forget it's name) to encrypt one of his drives, and he wants to move his OST file to this volume
Is this possible?
Yes, I am guessing it is possibly Truecrypt he is using.
By using Truecrypt, the drive should be visible as any other drive so from within the OS, it should just be unencrypted normal space that he can do anything with.
It should be able to be done - after all, unlocking the encrypted drive will present it as a normal volume just like any other.
To do this, unlock and open the encrypted drive, and start Outlook. Add the OST file as default, etc.
Now, your client will have to watch out for a couple of things. Firstly, as long as Outlook is open, the encrypted drive should be unlocked and open. Data corruption might occur if you lock the encrypted drive while Outlook is open. Secondly, there might be somewhat of a slowdown - especially if your client deals with a huge amount of mail.
If he can live with the drop in speed, as well as the inconvenience (and danger) of making sure that the encrypted drive is always unlocked before starting Outlook and closing Outlook, it should be fine.