I have two USB hard drives connected to the rear USB ports of my HP Microserver Gen8. I have enabled passthrough for the USB controller on the ESXi host, and have added both drives as USB devices to my Linux guest (Lubuntu 20.04). The problem is that the drives are not visible from within the guest O/S.
I tried adding them to a Windows Server 2008 guest on the same server and had the same problem - they were not being presented as hard drives (or removable devices). They were simply not visible in the guest O/S.
I'm running an HPE custom build of ESXi 6.7.0 U3.
Any idea how I can get my guest OS's to see the USB devices as disks?
You won't be able to get this to work this way, not easily anyway. ESXi was not designed to have USB drives attached and connected through to VMs. What's the purpose of the USB drives? If it's data that needs accessed by the VM's, then you should move the data into those VMs, not attach to unreliable external storage. If you are using it as some type of sneakernet transport, then you are better served copying the information to the USB drives while they are attached to a workstation.
I had a breakthrough. I removed the USB 2.0 controller from the VM, and added a USB 3.0 controller instead. I then re-added the two USB drives as USB devices and they are both now showing as drives in the VM.