I have a late 2015 iMac with 27 inch 5K Retina, 3.1 Ghz Intel Core 15, 16 GB memory. I have a 4-port USB expander in one of the USB ports. How many external hard drives (e.g., WD Passports) can I run at one time? Thanks.

Probably not more than one full-sized drive unless each drive is self-powered. A single USB port supplies enough power for one full-sized bus-powered drive. Your model may have one high power USB port that could support two such drives. Smaller laptop drives use less power, so the above will probably change to using two of them.

The best you can hope for is to plug in the drives one at a time until the port delivers an error. If you use self-powered drives then be sure to turn on their power and spin up fully before turning on the computer.

Provided you aren't using your USB Ports for anything except external drives and the 4 Port USB Hub, you should technically be able to run 7 External Drives. With that said, you'll probably run into some power issues with trying to run that many. I routinely have 3 or 4 that are plugged in, but several of them are self-powered.

Source: Math. (iMac: 4 USB + 4 Port Hub) - 1 USB for the Hub to plug into = 7

I have an older 7 port Belkin powered USB hub and had as many as 4 portable drives connected to it at the same time without any problem. (my list of portables includes, 4 Sabrent's, 2 WD Passport's and 2 Seagate Go Flex's) It will also charge all of my devices including (although slower than the Apple charger) my1st iPad.

My advice is to buy and use a brand name power USB hub, if you plan on having three or more portable drive connected to it at the same time.

for example > https://www.amazon.com/D-Link-DUB-H7-7port-Usb-2-0/dp/B00DMLTG3G/ref=pd_day0_147 _6?_encoding=UTF8&pd_rd_i=B00DMLTG3G&pd_…

I'd like to see 7 full-sized bus-powered drives running off one USB port with a 4-port hub. You could not run 4 such drives off the one port. 🙂