Same problem here on iMac Pro with four USB3 hard drives: two 3TB WD in Raid0, a 4TB WD my passport and a 3TB Lacie (used for TimeMachine). I noticed my 2 other ssd drives which use the thunderbolt ports don't get ejected.
The drives get ejected constantly after only few hours of usage. It happens while I'm not using the mac and wake the display. I have set the sleep of the machine and drives to "never". All the drives are directly connected to the computer, no hubs are involved. After they eject, they don't appear in the Disk utility. Only solution is to restart (log out doesn't help).
I have done a SMC and PRAM reset.
I have also wrote Apple feedback, but hope someone provides a solution quicker. I wonder if the problem is apparent in the beta of the next macOS version...
stefantotev
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