Are the internal drives on a MacPro3,1 hot swappable? I believe SATA drives were designed to be hot swappable, but I imagine there might still be some hardware restrictions.

So if you were to unmount a drive, can you swap out a drive in one of the internal drive sleds while the machine is on?
In 12 years of owning Mac Pros, I've never tried this.

MacRumors, however, says a resounding "No". Don't do it.

Apparently the protocol supports it but the hardware must too. XServes supported it, Pros don't.

Relevant threads…
Can you Insert Drive While Running Mac Pro ??
How to rescan SATA ports without restarting?

I also found on ArsTechnica - Did I fry my Mac Pro?