USB HDD not seen by Win7 64

I have three machines running this OS. Two see this HDD (Lenovo) and read/save to it. The third sees it in Drive Manager/Device Manager with no errors but will not will allow me to allocate a drive letter. I have a similar USB HDD (Samsung) which works perfectly well in all three machines. Disk Manager tells me that the the 'Disk Manager console view is not up to date...' when I try to allocate a drive letter.

I have seen many 'fixes' but none that quite match these symptoms.

Windows tells me that the driver is up to date and I have uninstalled/reinstalled. So, it would therefore seem to be a Windows problem and not a device problem but any suggestions are welcome.