TimeMachine backup shared network drive not listed under “select disk”

Question

I will use a shared network drive for TimeMachine backup. I can mount the drive via the Finder: Network -> MachineA -> Drive0. Get Info shows the drive is mounted as: afp://MachineA._afpovertcp._tcp.local/Drive0. This is the same way a second drive (call it Drive1) is mounted from a second mac server (call it MachineB). When I attempt to select Drive0 as the TimeMachine disk, it is not listed as an option, but Drive1 is! Incidentally, MachineA is currently using Drive0 as a TimeMachine backup disk. Call the client (to-be-backed-up) computer, MachineC.

What is required of a drive from a networking perspective for it to be listed as an option when "Select Backup Disk" is invoked? In other words, what do I need to check/ensure about the connection between MachineC and MachineA such that Drive0 is mounted/discovered in such a way that it will be listed as an option for selection in the TimeMachine preference pane?

(MachineA, 10.11; MachineB, 10.13; MachineC, 10.12)

Answer 1

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&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