Backup Disk Problem

Open Applications > Time Machine > turn off and remover that drive from the list.

Then try to erase and format the WD drive,

as per > https://www.hellotech.com/guide/for/how-to-format-a-hard-drive-for-mac

If you can not do that, then the drive is failing or has failed and needs to be replaced.

For a decent replacement Time Machine drive, do yourself a favor and get an external desktop drive

like the > https://eshop.macsales.com/shop/owc-mercury-elite-pro

the > https://www.amazon.com/LaCie-Professional-USB-C-External-STHA4000800/dp/B07G8JT7XN/ref=asc_df_B07G8JT7XN/?

or the > https://www.westerndigital.com/products/external-drives/sandisk-professional-g-drive-pro-thunderbolt-3-hdd#SDPH51J-004T-NBAAD

What version macOS? What hard drive is it?

Have you tried reformatting?

That would be my first step.

👌🤞

What year iMac? Have you tried a different USB port?

Yup, but try @den.thed’s steps above. For a no-fuss, solidly reliable, value-for-money drive, I recommend the Toshiba puppies https://storage.toshiba.com/consumer-hdd. Good luck!