So, for example, if you want to have 1TB of space available for Time Machine, you must write out “1000000”. While most other areas of the unRAID GUI allow you to simply type “1TB” or “500GB”, this particular section must be written in terms of “MB”. Set your “TimeMachine volume size limit” to whatever you feel is enough to give you backup capacity for however long you want your backup history to go back.Under “Export” select “Yes (Time Machine)”.Next we need to set the AFP Security Settings. I’ve seen some reports of people saying that Time Machine works just fine with cache disks enabled, but for my particular setup, I only have a single cache disk at 500gb that isn’t quite large enough to hold the 600+ gb of data being transferred during the initial backup from my MBP. Keep in mind that you can still use the same disk for other shares without any issues Under “Included disk(s)” select the disk in the array that you’d like to use for your Time Machine share.Under “Split level” select "Manual: do not automatically split directories”.
Set your minimum free space to whatever you'd like (I set mine to 10GB as I rarely have over 10gb of data that I need to transfer between backup intervals).I just keep it at the default "High-water"
Quick note: if your Mac ever asks you for server credentials throughout this guide, make sure you always check the box to save the credentials for future use. Before you begin, make sure AFP is enabled as a share option by going to Settings > AFP under “Network Services” > set “Enable AFP” to “Yes” and click “Apply”. I've yet to find a complete guide on the subject that has worked for me, so I've decided to create a step-by-step guide that has worked for me on the two systems that I'm currently running. I've been using unRAID now for the past couple of years, and only recently have I finally managed to properly setup Time Machine.