iMac keeps ejecting my Time Machine

What external hard drive are you using and how old is it?

It could be any number of things, a loose cable, a failing chip in the enclosure or the drive is beginning to fail.

If you are using a USB Hub, try plugging the backup drive directly into a port on the back of the iMac.

Next I would try replacing the USB cable with another cable, if the drive has a removable cable.

You have to many other USB devices connected to the iMac that are using to much power.

If none of those help, then it maybe time to replace the backup drive with a new drive.

Try these settings

The OP is running High Sierra.

I found this old OS X screenshot that I had saved years ago.

It was highlighted for preventing the iMac from starting up after a power failure.

What USB HUB? Is it self-powered or bus powered?

For some example's of self-powered hubs,

see > https://www.amazon.com/Powered-Usb-Hub/s?k=Powered+Usb+Hub

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Yea' you should use a self-powered hub for connecting multiply bus powered devices like portable drives.