Data size doubled when backing up

Hi everyone.

Im running a backup with the Zimaboard 2 on ZimaOS v1.5.4 from my mac. The backup is larger on the HDD’s on the zimaboard than on the macbook itself. Total drive size is 1TB and backup is now roughly 1.1/1.2Tb in size.

Not sure if this behavior is intended or a side effect of running the drives in RAID 1 (something going wrong with drive mirroring???). Bit of a NAS noob here so potentially just missing something obvious.

Happy to provide further details if required for troubleshooting/bugreports.

Thanks in advance.