Thanks for all the advice. The HD, an WD My Passport 1TB hobby from 10 years ago had 2xFirewire, 1x SATA and 1x misc elongated 'v' port to which I found a cable (glad i kept all those older cable for a rainy day!). This enabled the HD to be read and I transferred old photo files to a more modern (and much smaller) HD. All good so far!
Due to the extremely physical nature of this I fell asleep and had a nap. When I awoke from my slumber, the still attached HD wasn't showing up anymore. I did the usual checks to see what was what, incl Disk Utility (Mac OS). Strangely the HD shows up in Disk Utility, but not on my Laptop's locations section. All that is shown is the total capacity 1TB approx and that the HD is 'uninitialised'. Also when I connect the HD each time a box appears that says it is not readable on this computer.
First Aid runs in a second and says all well, but obviously it isn't. Is the remaining 600GB gone/lost/irretrievable, or is there a chance of simple recovery ? As always any pointers appreciated.