external hard drives

Regardless of what drive you purchase if it comes with software on it do not install or run the software. Just erase the drive and format it as APFS or MAC OSX Extended.

I agree with Redbrickman on the OWC drives. They have many different modules which are very reliable. If you need the drive for Time Machine then these are excellent choices: OWC Mercury Elite Pro with USB 3.2 or OWC Mercury Elite Pro with USB 3.1 Gen 1 + eSATA + FW800 depending on the age and model of Mac you have.

For your model contact OWC's customer support and get their recommendation for the drive model that would best suit your iMac model and needs.  They may have additional models other than those I've shown above.

Welcome to Apple Support Communities.

I like the SanDisk Professional G-DRIVE - Western Digital. It comes pre-formatted as Mac OS Extended (Journaled), and you can easily convert to APFS if you’d like.

Jack

I have some additional comments. These days, most external drives are compact, reasonably priced, easy to install with vendor provided USB cables and most do not require external power. The solid state drives (SSDs) are very fast but the conventional HDs are cheaper. I have had good service using the newer LaCie drives, but there are plenty of other fine drives out there.

Personally I recommend you avoid the Sandisk Extreme Pro V2 SSD drives. I have had nothing but problems with it and the fastest speed on a new iMac 24 has been 780 Mb/sec, but they are supposedly capable of 2000 Mb/sec. I was advised in this communtiy to consider the OWC Envoy Pro SX and although it is a Thunderbird only drive the reviews on it are solid with fast speeds.

All external drives are easy to setup. Just need to format or buy one already formatted for a Mac. as Jack-19 stated.