Hi
I re-initialised the Unit and it didn't make any difference....I've subsequently managed to resolve the problem...I bought a brand new identical replacement Disk which hadn't been formatted before, I've installed and formatted it on the unit without a problem and the unit has been working without a problem since...I will continue to monitor it over time especially whilst I reload the data in bite size chunks over the next few days.....
The replaced Disk was reinstalled in one of my PC's as a secondary HDD and its working well...so problem solved or should I say unsolved but with a work around!!!!
and many thanks for all your support...