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I chose Synology DS918+ for a home office network storage device, but it came with a drawback: only two USB ports. I needed to attach four USB devices to fit my backup strategy: An external USB hard drive (Seagate 4TB Expansion USB 3.0), an external USB hard drive dock (Thermaltake BlackX), a USB attached battery backup (APC 1500VA),a flash drive (anything USB 2.0 will suffice), and of course, a USB hub.
From what I’ve read and what has worked for me, the key is to use a USB 2.0 hub. Sure, you’ll have to sacrifice transfer speeds to USB 3.0 devices, but that may be an acceptable trade off to you. I dusted off a powered Belkin hub that’s been sitting in storage for almost a decade and it has been working perfectly. While mine has been long discontinued, this hub should work just fine if you’re in the market. I have the USB 3.0 drive plugged into one port and the hub plugged into the other.
Here’s what everything looks like in DSM: