About a year ago, I built an OMV server (using whatever ISO was current at the time) on a Lenovo M93 PC with a SATA attached RAID array, which works well, but I could not get it to recognize any USB-attached disks. I thought I had done something wrong, but since the system was otherwise working and had enough storage, I didn't bother to investigate further.
Today I am starting a brand new build on a Dell Optiplex 9020 using the most recent ISO (IOW, probably not the same version I used last year), and I am experiencing exactly the same issue. The system recognizes the boot drive and one additional data drive (both SATA III) but when I connect my two external USB drives, nothing. fdisk -l doesn't show them at all. However, If I connect them to the USB 2.0 ports instead of the USB 3.0 ports, they're recognized almost immediately by both fdisk and OMV. I tried disabling the USB 2.0 ports in the BIOS, but that didn't help. I'm in kind of a hurry here, I have about a day to get this finished and all my media copied. Thanks for any ideas!