unless something goes very wrong, all you should notice is some degraded performance. It's raid10, which seems to rebuild faster than our stripes of two raid1 sets, so hopefully it won't be as bad as usual.
edit 15:50: the raid is rebuilding.
md1 : active raid10 sde2 sdd2(F) sdc2 sdb2 sda2
955802624 blocks 256K chunks 2 near-copies [4/3] [UUU_]
[>....................] recovery = 0.1% (555264/477901312) finish=315.1min speed=25239K/sec
I note Coins also has a bad disk. I will be replacing that one as soon as I can run back to the office and get another spare.