I just had my first HDD failure with the RAID-5. One of the 1.5TB Barraducas (ST31500541AS) dropped dead. I've swapped the drive and added the new HDD to the array with:
sudo mdadm /dev/md0 --add /dev/sdh1
The raid is now rebuilding itself:
Personalities : [raid6] [raid5] [raid4]
md0 : active raid5 sdh1[3] sdb1[0] sdg1[2]
2930276864 blocks level 5, 128k chunk, algorithm 2 [3/2] [U_U]
[=>...................] recovery = 8.0% (117313844/1465138432) finish=516.8min speed=43460K/sec
unused devices:
This is the critical part. If the rebuilding fails due to failure in the remaining good drives I will lose the array and all data within. If it was RAID-6 I could still lose a 2nd disk without data loss.
Obviously I've backed up all important data before the rebuild, so only some less important data would be lost if 2nd disk failure should occur.