Soma again, Thank You for the input and guidance
If it helps anyone in the future :
I have a Pi400 4gb with the Raspberry Pi OS ( with desktop ) running.
My source drive was 500Gb
Target drive was 480 Gb ( SSD in a small USB 3 case )
On the Pi, I used System > Add / Remove Programs
Searched for, and Added gparted to the Pi
Searched for, and Added Clonezilla to the Pi
Used gParted to reduce the source drive partition to 460 Gb
Opened Terminal, and ran : sudo clonezilla
I first tried the 'Local Drive -to- Local Drive' option, but it failed as the target drive was smaller.
Then used the 'Local Part -to- Local Part', to clone the main partition from the source drive to the target drive.
Process took around 3 hours to complete ( maybe should have been faster - see problem below about drive connection )
I then tried a reboot, and could not get a reliable connection to the new drive
Connection was erratic. It would appear, then disappear.
Tried using a powered hub for the drive - same problems.
Swapped USB cables out with another identical case - same problems.
Swapped the SSD to another identical case - same problems.
Did some google-fu and found that the cases / adaptors that use the JMS578 chips, are problematic when used on a Pi. All my cases are identical and all have these chips in the adaptor.
Eventually stumbled on this page :
https://forums.raspberrypi.com/viewtopic.php?t=245931
Did the changes in the article, and 'bingo' - drives load, fast to navigate and read / write, etc.
I understand that the changes made do have an effect on drive speeds, but considering I have 4 of these enclosures, and not looking to buy more ( different brands may also very well have the same chips in the adaptors ), I am going to go with this solution.