G-Drive daisy chain HDD ejecting

I have 3 G-Drive HDD’s daisy chained directly from the Thunderbolt 3 port of my 2019 iMac. I am having random "disc not properly ejected"error messages but it only seems to be affecting one of the drives. They are all correctly linked with thunderbolt 3 cables, the iMac to the 10TB, to the 6TB & then the 4TB with a small 2TB mobile unit and of the line, issue I have is that it only affects the 4TB drive ?
I have tried reformatting it, swapping the sequence about between the 3 G-Drives but all to no avail, it seems to be more prolific if the iMac goes to sleep. I have just updated to Big Sur and prior to that I was just running the 10TB & 4TB with no issues but added the 6TB for my Photography/Lightroom stuff. Anybody any ideas ?