By MHO SyncToy has flaw in multithreading. To avoid the problem of 99% CPU load and 15 GB of ram used just start Task Manager, go to the process, right-click and Set Affinity, give the process no more then two cores - ltes say CPU 0 and CPU 1 - and everything will calm down to normal.

I use SyncToy for half an year and I am happy with it. This is the only minor glitch. I sync two of my laptop HDDs to external HDD during work and have no problem - more then 300 GB of information.