Raid failure, why?

[Michael Hipp]

This happened almost exactly a week ago. Now it has happened again:

$ cat /proc/mdstat
Personalities : [raid1]
md255 : active raid1 dm-2[1] dm-1[0]
       1044160 blocks [2/2] [UU]

md3 : active raid1 hda4[2](F) hdc4[1]   <---
       136351616 blocks [2/1] [_U]       <---

md2 : active raid1 hda3[0] hdc3[1]
       1044160 blocks [2/2] [UU]

md1 : active raid1 hda2[0] hdc2[1]
       18787904 blocks [2/2] [UU]

md0 : active raid1 hda1[0] hdc1[1]
       104320 blocks [2/2] [UU]

unused devices: <none>

I can fix it right up with

    mdadm /dev/md3 -r /dev/hda4 -a /dev/hda4

And all appears well for about a week. But it seems bent on returning. 
Do I have a looming hardware problem?

I've grep'ed every log file for any mention of md3 and utterly nothing 
out of the ordinary.

Thanks,
Michael