Android disk encryption - using as a removable device I'm about to upgrade my Desire HD to ICS so that I can use the full disk encryption functionality.

Does anyone know if its possible to still use the device as a removable disk when plugged in via a USB cable?

If this is possible, am I right in assuming that the device will need to be unlocked with the PIN/password before being mounted (as this surely defeats the purpose of it, if you can access the SD card without unlocking)?
In the official HTC ICS ROM for HTC Desire S, it is possible to encrypt the SD card in addition to the internal storage. It seems likely that the same would be true for the HTC Desire HD. I haven't been able to find details of how this encryption is done, yet. 

With an encrypted SD card it is possible to mount the device as a removable disk when plugged in via a USB cable -- I do this myself for backups. 

I assume that the SD encryption uses the same passphrase as the internal storage encryption as no other passphrases are required. It could conceivably be done using some internal passphrase invisible to the user, though I would be surprised.