Time Machine backing up to WD My Passport For Mac

Time Machine on my 2021 iMac M1 running Sonoma 14.0 backs up to a 4TB WD My Passport for Mac that I have used since I bought the iMac. Time Machine backups are made and I can restore files from backups, so setup seems fine but there is one big issue:

If the iMac goes to sleep it stops doing regular backups and when I try to manually backup I get the error message:

Time Machine couldn’t complete the backup to “My Passport for Mac”

The backup disk is not available. Make sure the backup disk is connected or select a different backup disk.

The WD icon is still on the desktop and I can double-click to see all the files but I can't re-establish the connection without restarting the iMac.

Am I the only one experiencing this issue?

Is there a known fix or workaround?

Do I keep restarting my iMac every time I want Time Machine to work?

TIA

Ian Jenkins

Sydney, Australia