A opção -W está implícita se você usar rsync sem copiar de / para um sistema remoto (isto é, somente entre duas pastas locais):
-W,--whole-fileWith this option rsync’s delta-transfer algorithm is not used and the whole file is sent as-is instead. The transfer may be faster if this option is used when the bandwidth between the source and destination machines is higher than the bandwidth to disk (especially when the "disk" is actually a networked filesystem). This is the default when both the source and destination are specified as local paths, but only if no batch-writing option is in effect.
Tente executar com --no-whole-file ou --no-W :
rsync -avhiu --no-whole-file --progress --stats folder1/ folder2/