@JorgeB Thankyou for your time - I've solved this 1, Basically it was due to bad naming of the zPool.
I used spaces in name when formatting in Unassigned Devices, once mounted, Unraid placed underscores in place of the spaces which then lead to the issue of not being able to unmount with the error being "Cannot determine Pool Name"
It also stopped the system from rebooting which resulted in a hard reset (i'm guessing due to the zfs pool hanging)
Once I removed the spaces my next big mistake was naming the 1st dataset the same name as the zpool device, this lead to all the same issues I had before (unable to mount this time and again could not reboot, stuck on "syncing filesystem")
Now that I named the zpool "ZFS--BACKUP--SSD" and my dataset to "Cache-mirrored----Backup" I can mount and unmount until the cows come home!
