I thought it was just Samsung that didn't allow SD cards to be formatted this way. Motorola has joined the party. Thankfully there is a workaround.

Enable developers mod on your phone by going in "Settings" -> "about this phone" and tap 7 times on build number.

Go to "Settings" -> "system" -> "advanced settings" -> "developers option" and enable usb debugging.

Connect you phone to your computer.

BACKUP YOUR DATA FIRST. What you will do will erase your sd card.

Download the motorola drivers for your phone here and install them (I dont know if you really need this but I haven't tested without them).

Download the Android SDK for your platform here an unzip it.
Open a shell in the folder where you've dropped the sdk files.
Your phone must be in "No data transfer" mod.

Tap the command "./adb shell" in shell. A message will appear on the screen of your phone. Accept the usb debugging. If you have an error after this command, retry this step.

Tap "sm list-disks" to find the disk number of your sd card (for example I have "disk:179,0").

Now, you will use the command "sm partition" to FORMAT your sd card. You have two choices:

you format your whole sd card to internal storage by typing "sm partition disk:YOURDISKNUMBER private

you create a mix between internal and external storage by typing "sm partition disk:YOURDISKNUMBER mixed RATIO"

For example, I format my sd card 50% percent internal with this command : "sm partition disk:179,0 mixed 50"

Finally, disable usb debugging on your device (and dev mod if you want)