Just wanted to add what @Peacekeeper said, that if you are planning to use the Hidden Volume (HV) feature with size bigger than half of the Encrypted Volume (EV), you should not use NTFS file system, or any other which writes its backup in the middle of the volume, otherwise you risk damaging the EV with a HV.
I am not sure whether EXTx is supported across all OSes natively, and would go rather exFAT. One way or another, this is transparent to the device, as it is working on the volumes rather than partitions, and you can have any configuration there. This is true for Unencrypted Volume, EV, and HV.
In case you have used the SD card outside of the device please remember of overwriting it with random data (by invoking “initialization” procedure), so that encrypted data location would not be revealed by simple inspection.