Got hammered by Too Many Corpses. Reinstalled the macOS High Sierra and migrated data over from original HD to a new externally connected (USB3) 1TB SSD (APFS), original drive was also 1TB (HFS+ / MM Server). Tried to run Time machine (excluded all drives connected) and it estimated a 3.1TB space requirement. I get that TM needs Drive Size + 20% , the 1TB system drive is approx 80% (0.8TB) so even a 1.2/1.5TB estimate would be to surprising but 3.1TB!?! That seems excessive but perhaps it is not. I don't know. Maybe this is a APFS > HFS+ issue? I have noticed drive size estimated in finder are way off for externally connected machine. In my backup regime I will carry at least 2/3 clones of the drive but also want one Time Machine backup. I never want to be caught out by this. I don't think an update even ram but I did shut the machine down for the first time in months and on reboot, perhaps an update (not crucial) ran and borked, or I suspect malware bytes may have done something nasty. Hard to say. Tried some terminal fixes but they didn't work. No Data loss by the looks of it! Most important of all.