External Hard Drive recommendation

Also, you can find a number of drives available on crucial.com and mac sales.com. Many of the drives available these days are reasonably priced, compact, require no external power and easy to install with a provided USB cable.

A bus powered drive does not have its own external power source (an AC adapter/plug). It is powered via the USB connection. However, there are limitations as a USB port only provides a limited amount of power to the connected device. Many devices powered this way must be (they say) connected directly to the computer to receive sufficient power to work. They often fail to work if connected to the computer via an external non powered USB bus. However, many of them will work when connected via an external bus which has its own external power source. This is a good thing to keep in mind. It can save a lot of grief.

Bus power needs no AC power cable, it can draw limited mA from the USB cable, but is limited to 500mA in USB2, & 900mA in USB3, yet a decent external drive needs 1500mA to startup. that got around that by fancy trickery.

2015 iMacs have USB3 & Thunderbolt 2, so not as much worry for you on Bus Driven, & if speed is essential, then the ones on the right side here would work...

https://eshop.macsales.com/shop/external-drives/OWC-Envoy-Pro

These have ben my most trusted external Drives...

https://eshop.macsales.com/shop/firewire/1394/USB/EliteAL/eSATA_FW800_FW400_USB

What year iMac is it, if it just has USB2 then I'd not recommend Bus powered drives.