This can be covered with a cache drive, which is used to hold all the data as a buffer, and then, at a predetermined moment in time (I believe at 3:30 am), unRAID starts copying everything to the array.

I still have an 80GB Intel SSD laying around here which seems perfect for this job (providing I don't write bigger files than 80 GB to the array in one go, which should never happen after the initial backup has been made - incremental backups are not a problem). This means the writing speeds to the server would be a lot faster even than the read speeds (since I would be writing to an SSD )