@Zonk Welcome to the Rockstor community.
Note that the single btrfs raid profile, like the raid0 profile, is not a redundant profile. For that your best bet is really raid1 (which has a 2 disk minimum so really 3 disk is a nice practical minimum device count wise). Note that btrfs raid1 will try to ensure that there are 2 copies of all data on 2 independent devices (hence the minumum of 2 disks), even if there are 3 devices, ie it’s 2 way raid1 irrespective of drive count.
Yes the hot plug scenario is a little touch and go on the btrfs side, although if you could avoid using the usb bus, ie favouring SATA for example, you are likely to get better results as the USB bus is generally not recommended as highly as the SATA bus for such things as disks; especially in multi drive scenarios as it can often do internal resets that affects more than a single device at a time is my understanding. But then not all SATA ports are rated for hot plugging so bit of a catch 22 there. Although I think it was part of the original SATA standard you still get ports / controllers that claim hot plug capability while other don’t.
There is no difference between 3.9.2-26 and 3.9.2-27 that would account for this change in behaviour by the way so this is an underlying technologies type thing: not unless the kernel was upgrade at the same time anyway.
Changes per release version:
Maybe other forum members can chip in here, re the hot plug scenario / kernel panic / USB verses SATA bus etc as it’s not really something I’m that up on myself.
Also note that the missing disk / degraded pool warning in the Web-UI title bar is only updated every 30 seconds:
Hope that helps and thanks for sharing your findings.