SATA SSD fails to eject/Unmount

Currently using Prime 4 with 1TB internal SSD. Once controller in computer mode I can connect drive to Mac running Catalina and the disc appears on desktop. Disc utility is unable to unmount disc, nor able to check to see if there is a problem with it as it says there is a program running. I have tried ejecting in Engine Prime etc but no joy… SSD cards and USB thumb drives don’t have this problem. Drive works in SSD internal source mode no problem, just the problem with ejecting/unmounting. Currently I have to power off Mac to unmount safely. Is this a problem with the SATA drive or something Prime 4 is doing to hold it open?