From searching this forum and reading how to replace hard drives (and partition them) in WD My Clouds … it’s not that simple and not for beginners.
Firstly, WD My Clouds use EXT4 Format/Partitions which is Linux OS … My Cloud Homes would be no different.
There’s plenty of guides for WD My Clouds but not My Cloud Homes (see below)
Personally speaking … if you have haven’t formatted the original hard drive, i would put it back in the WD My Cloud Home enclosure and abandon the headache of attempting replacing it with a smaller hard drive.