Had one drive just up and die recently and needed to replace is, but forgot the steps within the NAS4Free GUI. Fortunately I came across another site that had the answer I needed, in a much simpler and understandable format.
So I followed their guide using PuTTY, accessed my NAS and viola, drive is resilvering and should be good.
Here’s the link to follow. http://www.bolsondigital.com/2013/06/01/replacing-an-hdd-in-a-zfs-pool-on-nas4free/#comment-2056
Addendum: All is well after the re-silvering. Good luck.