Expansion with failing drive

One of the 3 drives in my ReadyNAS NVX is dying. It was kinda hanging in there, just a few reallocated sectors, so I bought 2 new drives. I decided to add the 4th drive first, to do the expansion. Don't ask me why. Sometimes I'm not very smart, I guess.

In any case, it's been restriping for the last 10 hours or so (these are 1.5 and 2TB drives), and when I got up this morning, the ReadyNAS emailed me with a bunch of errors on that first drive (ATA errors, which I had never ahd before, and reallocated sectors), and the rate for restriping has dropped to 18MB/sec, with another 10 hours/51% remaining.

So. If the first drive completely fails DURING the restripe, am I completely screwed? I'm thinking that pulling the first drive and replacing it now is a HUGE no no and I don't want to do that unless it's the last resort.

Any advice? I've got most of a backup (not my TiVo recordings; those were still uploading to Crashplan when I started to do this, with 30 days remaning).