I'm using SyncToy 2.1 to backup my (Window 7) main system's main data HD to a same size HD on a secondary (XP) PC. That 1.3 TB HD is about 95% full, and sees about 30% of files changing per month, which is my backup frequency.
I'm doing an ECHO Sync, from my local D: to L: which is mapped to my backup PC's data drive.
I've checked, D: and L: have the same size (+/- a handful of megs).
SyncToy keeps failing with Disk Full errors. It does the 'checking" phase OK, then starts copying a bunch of files, but fails in the end.
It seems SyncToy is too conservative about deletions, and runs out of space for new files on the backup drive because it doesn't mirror the deletions from the source drive early enough.
I've gone back to doing xxcopy d:\ l:\ /CLONE... That's delightfully 90s, and I'm glad to see that good old command line again, but I'd still prefer SyncToy to work for me.