Debian Hardware :: Hard Disk Vanished - Should I Be Worried?

My system currently runs mdadm with three mirrored volumes (4 disks, two mirrored partitions on the first two (a) and (b), entire disks mirrored on the second two (c) and (d)), giving me some redundancy in case of disk failure, which it appears I now have. The root volume is mirrored on sda1 and sdb1, and this morning I noticed a mail from mdadm telling me the two RAIDs on (a) and (b) were degraded. It appears things went a step further and /dev/sda is completely gone.

mdadm has come through, though. The system is still running normally with the mirrored partitions on disk (b). What's got me concerned is that disk (a) is only about a month old and was bought brand new. I'm not at home right now to check the actual hardware, but seeing as the signs are pointing this way, do they necessarily mean the disk has failed? Is there something in the logs I could check that would tell me if, say, the PCI SATA card is playing up instead? I know hard disks fail, but I'm paranoid in case tings get worse as I'm running a reasonably important website on this machine, and just wondering if I should be considering setting up a backup server in case there's a bigger problem here. Edit: Sorry, forgot to say. I'm running Squeeze and it's fully up to date. Hardware is a 1.2GHz Celeron, 512MB RAM, Silicon Image SiI 3114 PCI SATA Controller, 2x 500GB SATA disks (a and b), 2x 1TB SATA disks (c and d).