Many thanks for your reply.
Yes, it is in NTFS. Was one of the first things I checked. I also checked all the connections, even though I can access the drive form any program that I have data stored, i.e. Word. Opened up the files OK. At present, I have totally disconnected and will check it out in a while, though this has been like it for some time now - at least 3 days.
If it doesn't work, it looks like I may have to install the new HDD first. Bit annoyed about that too. It arrived today, but no SATA lead was included.
I have tried to use the Buffalo in the past as a back up, but have always got an error message: 'Windows encountered an internal error. Review your back up settings. (0x81000027)'. so whether that has got something to do with it, or whether there is a fault with the HDD, I don't know.