Hi Sylvia
I'm having very similar problems. I have tried 3 different external RAID drives over USB 3.0. They all work well on both Windows and Linux when connected to my laptop. They all fail to stay connected in the long term when connected to the D54250WYK. If I set the spin down time on the Netstor NR710C RAID box to 1 minute on the box's hardware setting, the NUC can easily bring the RAID back to spin up, showing the Ubuntu 14.04 OS is correctly doing its thing. All working as it should. The OS is set to not power down anything USB related by changing files at GRUB level and through an application called TLP by Linunner.
BUT if the D54250WYK is left for an hour or so after the NR710C has spun down, it disconnects from the NUC. This disconnect won't happen if I set the NR710C hardware to not spin down, i.e. exactly as Steve described.
I can see the USB power lights for other connected devices, such as a USB TV Tuner drop as well after this period, and that usually means the RAID will be disconnected.
Like Steve, the OS is set to maximum power/performance. I have the latest 25 BIOS installed.
Given that I purchased the NUC as a low energy home server, having a RAID box permanently on and using power makes this an unsustainable situation.
I've set the NUC for maximum performance in the BIOS and have tried toggling both the ACPI and ASPM in the BIOS to no avail.
I bought the D54250WYK on the basis of a Intel paper on using the NUC as a HTPC with Linux, but it cannot currently function like this at scale.
I've also found similar complaints online. I would be very grateful to find out when I like Steve will get an update to the BIOS that doesn't drop USB 3.0.
Many thanks
Nas