Sync starts full copy.

[pjaj]

I use a USB memory stick to backup my main data drive (about 360Gb) every few days under Windows 10.
There should be about 1,000 files to transfer (around 1Gb) each time.
But occasionally, when I run compare, it says the whole disk needs synchronising which can take over 12 hours.
Any idea what causes this and how to prevent it?

[xCSxXenon]

https://freefilesync.org/manual.php?topic=daylight-saving-time ?

[pjaj]

Thanks, I'd completely missed that bit!

[pjaj]

Later
It's still not right. When I switch to my other USB drive which I use for backing up the C: drive it filters out 1.15 million files, but still wants to create nearly 430K files (mostly 116K from Users and 279K from Windows) and update a further 23K.
That can't be right.