Hi, in another thread I wrote about USB issues on my new 27" iMac. If you don't like to read the whole text, just tell me please what's your build number of El Capitan on the Skylake iMac. I wanted to disable the fusion drive, but was not able to boot the installer from an USB thumb drive, which I used several times before to install El Capitan on other Macs. It showed me an empty disk symbol in the boot menu. Then I tried to download El Capitan from the AppStore to create a new one, but it did not let me download, because El Capitan was already installed. On my MacBook Pro I can download it anyway and just skip that message. I though maybe it's why my MBP is on 10.11.1, so I updated the iMac too, but a download was still not possible. Also it was not possible to boot from the internal SSD of my MacBook Pro with 10.11.1 Beta 4. I connected it to an USB port and it failed booting after some time. So I started Internet Recovery on the iMac and installed El Capitan on another USB SSD. Finally I was able to boot from an external device to disable the fusion drive. After that I installed El Capitan on the internal PCIe flash storage, from the recovery partition of the external SSD. For not any special reasons, while installing, I read the detailed installation informations and there was an error that said something like a 10.11.0.1 package could not be found to download. The installation finished with no official displayed error and booted normally, but my USB SDD at least since then has only 2.0 speed and works only on the first port (from the middle). In the meanwhile I created a new USB boot device on my MBP and the iMac did not recognize it. I tested it on every port and also another USB thumb drive which has a Mavericks installer on it. None of them showed up on the Desktop or in Disk Utility. Updated again to 10.11.1, but that did not help me out. So, may it be there are special Skylake drivers for the new iMac, which are not included in the regular build of El Capitan and also not in the latest beta of 10.11.1 and Internet Recovery does not provide these drivers yet? Or is anything wrong with my iMac? Edit: The two USB transmitters for my wireless Logitech keyboard and mouse seem to work on any port. I am now testing a clean install with Internet Recovery this time from the internal drive again.