This is fairly vague at this point, but I have been able to reliably lock up my XU4 when doing heavy I/O load to a hard drive plugged into the cloudshell SATA connector. This is on 4.2-rc1-41.

The best way I've been able to do this consistently is by compiling the kernel on an SSD drive via:

$ make -j9 all

I've been able to reproduce on two different brands/types of SSD. It never happens when running the same command off an SD card.

When it locks up, the fan stays running and I have to unplug/replug the power cable. Oddly, the SATA drive won't be recognized until I reboot a second time. The only interesting bit of kernel log after the first reboot is:
usb 4-1.2: reset SuperSpeed USB device number 3 using xhci-hcd

Again, sorry for the vague issue, but curious if anyone else can reproduce.