LEGEND - 100% correct - Thank you
I downloaded the drive manufacturer's utilities, mounted the drive in a USB caddy and completely reinitialised the disk (1TB took almost 12 hours to overwrite with 0's). Then ran a full set of diagnostics and extended scans, which took a few more hours. All clean
I also decided to download v17 - had been using v16 previously
ran the install onto a single 1TB disk - succeeded first time!!
added 4 more 4TB disks, formatted these as GPT EXT4, then configured these as raid 5 - all worked first time!
all working 100% - apart from usb drives not being recognised. If i attach a usb drive directly the to 5Big V1 via port at the back, it is recognised in disk setup > smart tab as sek, but does not appear in disk set up > usb disks. I'm still working on this. Could be something to do with the usb disks being mac formatted? and my pc is windows 10?
BUT - am extremely grateful - without fvdw-sl my server would be a brick - many thanks once again!