Will iCloud back up external HDD connected to my iMac?

No , iCloud won't back up external hard drives connected to Mac see the article Other ways to back up and restore files - Apple Support

Cloud-based services don't require you to connect an external hard drive to your Mac. For example, iCloud can automatically store your photos, calendars, contacts, notes, passwords, and more. And it includes iCloud Drive, which can store documents, desktop files, and other files of your choosing. To access those files, you just need to be signed in to iCloud. Other cloud-based services are available from third parties.

4231NadaDeNada Said:

"icloud: will icloud back up external hard drives connected to my imac?"

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Downloading All of iCloud: 

No. But, you can do it the other way around...Use my user tip: Getting a Quick and Direct "iCloud Drive-to-External Hard Drive" Download - User Tip

I have been testing the v1.6.68717 | Feb. 11, 2016 version of the Audio and Midi drivers.. with their Ultralites and so far no issues at all. I'm also hearing positive reports from 828 owners.

The AVB range of products already worked with 10.11.x

[Link Edited by Moderator]

No. iCloud is not a backup service.