Can hot swapping 3.5" / 2.5" drives happen to any OS , Mobo or case? I'm still deciding my hardware and like the idea of it but do you need any special Mobo (Asus I have in mind) that are set up for it like some cases are, ie one with eSATA or a cable having to connect from the back plate. I dread the thought of buying to find out I got the wrong stuff.
SATA has supported hot swap at the hardware level for many years now.
If you plan to use an external enclosure it is better if you have native eSATA ports, or (if you're going to use an adapter bracket to route an internal port to the outside of the case) at least an option in the BIOS to designate the port as external. The reason for this is that eSATA specifies different voltage levels on the cable, to make data corruption less likely when going through multiple connectors and/or longer cables.
On the software side, the driver for your SATA controller needs to support removable drives. AFAIK most (if not all) of them do these days.