Macos – Creating multiple Time Machine volumes for a couple users on Synology NAS

Of course it'll be best if you can get a big hard disk to use Time Machine effectively - however from your question, I am assuming that you have your reasons to not be able to get it.

In this case, I would recommend you to NOT use Time Machine, and manually backup. There are a couple of reasons for this :

1. You can selectively choose to backup only the most important data
2. The hard disk can be a single partition, and space can be effectively used.

If you have to use Time Machine no matter what, then I recommend you do this (if I read your question correctly):

1. Set aside 30GB for that Windows-use partition - That way you can use FAT32, and both Windows and Mac can write to it. It need not be large because I am assuming you are using it to transfer data from your Windows machine to Mac.
2. Dedicate the rest for Time Machine.

And please, have another TM backup disk once you are able to do so.

As I understand it there is a huge advantage for iSCSI that outweighs the issues above - it is seen as a local disk to the OS.

TimeMachine backups to USB, Firewire or Thunderbolt drives backup natively, while TimeCapsule and other Network drives get a 'SparseBundle" pseudo drive.

In my experience timeMachine on a sparse bundle always fails at some point in its life and has to be scrapped - especially over WiFi. I have had nothing but bad experiences using that system.

However (in theory) an iSCSI connection will use a native format (no sparse bundle) which seems much better.

If you need to restore you can always copy the Time Machine disk to a real USB or TB drive and mount it on the Mac - sure it is a pain, but really how often does that happen? Alternatively you can first rebuild the OS and reinstall the iSCSI driver, or boot off a Super Duper clone (assuming you have one).