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This suggests some problem with power or maybe controller.

Still not getting SMART report for cache. How is this drive attached?

Is that the full text of the error? Post your docker run as explained at this very first link in the Docker FAQ:

The proper place for your question is in the thread you already posted in. In any case, you shouldn't post in multiple threads about the same problem. How can we coordinate responses when you have multiple threads going? Go back to that other thread and ask again. Locking this one.

I found the other thread you were talking about, but it was never determined what the problem was since we got involved after you had already done some things and we never asked for more information. Bad connections are far, far more common than bad disks. Go to Tools - Diagnostics and attach the complete Diagnostics ZIP file to your NEXT post in this thread.

You shouldn't be using a RAID controller. That can cause a number of problems though that may have nothing to do with the speed you see. Go to Tools - Diagnostics and attach the complete Diagnostics ZIP file to your NEXT post in this thread.

Post diagnostics while the drive is fine.

^this

Go to Tools - Diagnostics and attach the complete Diagnostics ZIP file to your NEXT post in this thread.

It is allowed by the software, but not all possible hardware will work well, and all possible hardware in the case of USB external drives is very much a moving target, unlike standard SATA and SAS connections. Many hardware implementations of USB external drives have exhibited one or more of these problems: Non-unique identifiers, so Unraid can't determine which disk is supposed to be assigned to which slot in the array. Presenting drives with a non-standard size. This won't matter if not using parity, but parity can't be smaller than any data disk, not even by one byte. Not passing SMART information, so drive health can't be monitored by Unraid. Disconnects which causes a disk to be out-of-sync with parity. When this happens the disk is "kicked out" and has to be rebuilt. This will take several hours, depending on size. And it may happen frequently. This is why I said it might be OK without parity. But of course, without parity, you can't recover a failed disk. Probably others I am not recalling or not aware of. Another thing that is asked about is trying to use an external enclosure with a single USB connection for multiple disks. Parity operations are intended to work in parallel and if they can't performance is significantly affected. Much of this is unimportant for a single external drive that often isn't even connected all the time, which is the intended market for these designs.

https://www.youtube.com/channel/UCZDfnUn74N0WeAPvMqTOrtA

Apps - Settings - Enable additional search results from dockerHub

If you get your array started and still have problems post new diagnostics.

And be sure to boot from a USB2 port.