The “VDI Not Available” error appears when a virtual machine (VM) is shutdown inappropriately or if there is a concern with the Storage Repository (SR) where the virtual machine exists.
It normally happens because of storage connectivity issue or where in the VM was hard powered down. Since in such scenarios the host is not able to gracefully release the RW lock on the disk making the disk unavailable for reuse. To fix this issue we need to first release the RW lock on the disk.
Error code: SR_BACKEND_FAILURE_46 Error parameters: , The VDI is not available [opterr=VDI 86012149-349d-469d-969d-6ff4ee36595f already attached RW]
For a fix, see: Error: “VDI Not Available“
Ellis Karim's Blog 
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