By the way, I have been reading the user manual and there are a few interesting points: in no place is mentioned an internal HD, but it's specifically said that an external HD can be connected through the USB port; speaking of which, the PA-900 has two of them (one "host" and one "to device") and they are both 2.0, while in the PA3-X the connection to device is only 1.1 (hence the painfully slow transfer rate between internal HD and PC).
But, as far as the storage devices go, here is what the manual says:

Name Media type

DISK [KORG DISK] User-accessible area of the internal memory. This is where you can store Songs and other files.

SD [KORG SD] Optional microSD card inserted into the rear slot.

SD [KORG SD2] Optional microSD card inserted into the rear slot
(second partition).

USB memory device (like a memory stick) connected
to the rear USB Host port

So, compared with the PA3-X, looks like they added a few things but took away others; another important difference, for example, is in the sampling section: you can load samples and edit them but you apparently cannot sample your own sounds. So, more than a full fledged sampler, the one in the PA-900 looks like a sample reader, more or less like the one in the Tyros 4.
The big advantages of the 900 are the low price and the low weight: combined with some features, more up-to-date than even the PA3X, they make the whole package really interesting.