WD MyCloud Home Offline - Why?

Use Rescuezilla, it is free. Use sata instead of USB if possible, try image clone the original hdd and see if that can be done, avoid directly editing the original hdd. The target HDD has to be identical or larger than the original and since the 6TB WD hdd in the MCHome is likely to be SMR, that means a CMR 6TB won’t work because a SMR is nearly always a few sectors larger than the CMR counterpart. The image clone will take a few hours and if it is successful, there is a chance it could boot in the MCH. The MCH boot sectors should not be edited but the data containing partition 24 could be shrank or expanded at a later time to fit an alternate target hdd.