TimeMachine backup shared network drive not listed under "select disk"
The problem results from this detail: MachineA is currently using Drive0 as a TimeMachine backup disk.


  Note that the TimeMachine backup volume is "read-only" only for everyone user, but "read&amp;write" for system and wheel users. https://discussions.apple.com/thread/8216189


Thus, Disk0 was being shared as read only and hence the problem. 

Looks like there is a fix for this in 10.13, a location can be shared as a TimeMachine backup destination:

https://support.apple.com/en-us/HT202784

http://www.nandakusumadi.com/blog/2017/12/10/how-to-setup-shared-time-machine-drive