The now selling Seagate Backup Plus and the WD "My Passport," for example, require Mac OS X 10.11 and OS X 11, respectively.
That refers to the software that ext drive manufacturers want you to install. Most is garbage and need not be installed. Time Machine and basic transfer functions work fine without the dreckware the manufactirs foist on users.
For whatever it's worth, I read this at PC Magazine: "The only case with hard drives where the USB standard matters much is if you connect a drive to an old-style, low band-width USB 2.0 port. Also, if it's a portable drive, that USB 2.0 port may not supply sufficient power to run the drive."
There is some truth in that. "Name-brand" drive enclosures (not the drives inside them) tend to be designed to go on sale every other weekend rather that give long, reliable service. Also, as electric motors (like the one that spins the drive's platter) age, they can begin to demand more power. As a bus-powered drive must get ALL power from a USB port with power limits, that is a common point of failure.
Also those "portable" drive have low-speed 2.5-inch laptop-class drives that not very robust. The ONLY hard drive failures I've work in over 30 years of using computers with hard drives were with 2.5-inch laptop drives. When I worked in law enforcement, we called that a "clue."
If you must buy a cheap name-brand drive that goes on sale every other weekend:
- Use Disk Utility to erase and reformat the drive as Macintosh HFS+ (for your OS version). Most come with Windows formatting or special proprietary formatting that seems more likely to belch up a low-power problem.
- Buy a POWERED USB hub (comes with its own power supply) and place it between the cheap drive and the computer to take the load off the USB port.
My approach? After suffering grief with cheap bus-powered drives on our Macs and those I support, I finally asked, "How much is my data worth?" and decided to buy a better product. For our desktop Macs I now use only OWC Mercury Elite Pro external drives that contain proper desktop-class 3.5-inch drives. They have a an independent power supply so the low-power issues is eliminated.
This is it: https://eshop.macsales.com/item/OWC/ME3NH7T01/
The iMac compatibility list in that web page goes as far back as Late 2009 iMAcs but I've used that series on Macs as old as a G4 PowerMac from 2003.
That's a pro-class drive and most of the long-serving contributors here recommend them as well. As desktop computer has little need for a "portable" drive like the WD PAssport, go pro. Our oldest OWC exteranl drive is now between 15 and 20 years old and, although now relegated to light-duty due to great age, still works as wella s it did when new.
Disclosure: I have no connection OWC or its subsidiaries other than as a customer.