I'm new to unRaid, still learning if its right for me so decided to set it up on my old i7 860, I dont have spare ram just yet, so its currently borrowing 4gb from my desktop.

It also has 2x2tb in the array, (no parity yet as I eventually want larger drives in there and don't need the redundancy yet as I still have all my data elsewhere) and a 120gb ssd as the cache drive

 

Anyway, while transferring anything over ~100gb (the size of the ssd cache) windows halts the transfer midway though and tells me there is no space left on the drive.

 

I can wait (/come back hours later), and I guess mover is doing its thing and it begins copying again, but very annoying when you want to copy a tb of movies or game isos etc.

 

Is there some obvious fall-back setting i should have enabled or something?

Global Share Settings - Cache Settings - Min Free Space

You set the Min Free Space to be larger than the largest single file you expect to copy. When your cache free space gets below that any further writes go directly to the array instead.

 

You can manually invoke mover any time you want.

 

Also, without parity there is probably little or no advantage writing to cache unless you have 10Gb ethernet since your spinners can mostly keep up with your network if parity isn't being written.