The most critical factor in selecting drives for RAID:
"do they offer advance-replacement RMA service?"

In the UK that is Samsung - dodgy drives... great service.

As for the other questions : http://linux-raid.osdl.org/

checkraid essentially does :
echo check > /sys/block/mdX/md/sync_action

If however you do
echo repair > /sys/block/mdX/md/sync_action

Then it will read all blocks for a stripe and if a read-error occurs, will calculate the block from parity and attempt to write the block that failed to read.