Time Machine stuck preparing backup OSX 10.13 after Safari update

Time Machine fails to move on from Preparing Back up - I have iMac Mid-2010 running High Sierra. I updated Safari to 12.01 a week or so ago and then there were operating problems - running slowly etc. But worst of all Time Machine suddenly starting sticking on Preparing Back up. I had two external HDD running Time Machine - both doing the same thing. I got another external drive which had been formatted - that one backed up without a problem - took a while of course - but it only backed up the once - then it started saying Zero KB - checking the time or similar. So now none of my three external drives will move beyond preparing back up. I have a MacBook Pro of similar age in which I also updated Safari and that is running High Sierra - there are no Time Machine problems. It seems to me - not being an expert - that something may have got into my iMac as well as the Safari update. Do I reinstall from one of the Time Machine drives? But maybe what's causing this is in all three of them now? Should I reinstall from an earlier date on the Time Machine drives?