Please help, this is driving me crazy. I recently bought a WD My Book Thunderbolt Duo 8TB (software RAID 1 mirrored, 2 x 4 TB WD Red drives) to use with my Mac Mini (OS X 10.11.1). Until last Friday, the setup worked well as expected. But suddenly, the Finder didn't show the drive anymore and iTunes, Capture One and other software couldn't access it either. In disk utility, the drives show up three times due to them being in a software RAID configuration (so it shows each drive once, the RAID itself once and the RAID mirror system once). When clicking on "First Aid", it tells me everything is okay. WD Drive Utilities itself says everything is okay, from the SMART status to a complete disk check. The RAID monitoring in Drive Utilities tells me both disks as well as the RAID system are all online and working. The enclosure seems to work well too, since the LED flashes correctly when doing a drive test and the Thunderbolt port on the back correctly transmits the signal to my Thunderbolt Display. The only sign that something is wrong is when I restart my Mac, I get an error message stating "The inserted medium could not be read by this computer." When scanning the drive with Any Data Recovery, it indeed is able to find every single file, so the data still must be there. Also, Apple Hardware diagnostics did not return any error with my Mac Mini. Using the "diskutil list" command, Terminal correctly shows both hard drives in the RAID enclosure as well as the mirrored RAID itself. Using "ls /volumes", the Thunderbolt Duo does not appear, however. Any ideas what is happening and how I can fix it?