FYI, it appears that the newer versions of CrystalDiskMark v7 and v8 do not see the MCH kddfs and that v6 has some benchmark problems with the MCH kddfs, so what is left is that CrystalDiskMark 5.2.1 or 5.2.2 may be the best candidates for the job. You can download the old versions from here: Release CrystalDiskMark 5.2.2 - CrystalDiskMark - OSDN
If you see any other disk benchmarks that can support testing of the private space, feel free to post the name and results.
The example below shows that using the ‘Select Folder’ feature, you can see the TestDrive(Y:\Share1) which is coming from the WD Discovery mounted MCH. This particular MCH has it original WD40EFAX HDD replaced with a SSD drive (MX500). It is not unusual for random writes to be 0.000 for the original HDD drive in this test.
Comparison of SMR hard drive WD40EFAX to SSD (MX500) in a My Cloud Home
Dramatic slow down of performance of SMR recording with sustained random write
