Thank you der_bud, further investigations came up with the expanded error message - udisks error quark, 0 - but although this brings up much more info on Google, nothing I did corrected the problem. Then with a leap of female 'logic' I unlabeled the drive, and then labeled it again (with the same label), and it worked! I have no idea why, but I can now read the data stored on my second SSD harddrive.
Meanwhile I have learned a little more command line, so hey ho it wasn't time wasted ;-)
Thanks again,
Sue.