Drive 4 drive slot sensor drive fault detected?

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Drive 4 drive slot sensor drive fault detected?

I have a Poweredge 6650 that has been running fine with three 72GB drives in a RAID-1 stripe set. It is only powered on when needed as it hosts several VM machines that are used to test and verify things. Anyway on boot up today it complained about a failed drive and just my luck I only created a RAID-1 stripe set so no redundancy.
Anyway the data was backed up so not too big a deal. I pulled the failed drive and re-inserted it and it was recognised but showed 'Failure Predicted', so I brought it online. Through OpenManage I removed the old Virtual Disk and then created a new RAID-5 set (so only 136GB instead of 204GB). I then pulled the Failure Predicted drive and replaced it with a spare.

Slot Sensor Drive Fault Detected Free

It is now rebuilding (taking ages..) but there is an error showing that appeared 25-seconds after the replacement was installed:

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Slot Sensor Drive Fault Detected In Windows 7

I find it hard to believe the replacement drive is also faulty so is it possibly something else? I am not sure what the 'drive slot sensor drive fault detected' error actually means?
Cheers

Slot Sensor Drive Fault Detected Location

Andy