I built my first RAID array about a week ago, kinda vaguely understanding what I'm doing. 4 3 TB drives in a RAID 5. It's been working fine, and I've been slowly filling it up. Took the machine apart today to upgrade case cooling and power supply. When I brought it back up, I got this in the boot messages; md/raid:md0: raid level 5 active with 3 out of 4 devices, algorithm 2 My 1st thought was that I didn't get everything hooked up (I had to disconnect most of the drives to get the power connections on the board), several reboots later, I'd confirmed that the system was seeing all the drives; root at debian:/var/log# lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT sda 8:0 0 931.5G 0 disk ├─sda1 8:1 0 14G 0 part / ├─sda2 8:2 0 37.3G 0 part [SWAP] └─sda3 8:3 0 880.3G 0 part /home sdb 8:16 0 2.7T 0 disk └─sdb1 8:17 0 2.7T 0 part └─md0 9:0 0 8.2T 0 raid5 └─md0p1 259:0 0 8.2T 0 md /srv/media sdc 8:32 0 2.7T 0 disk └─sdc1 8:33 0 2.7T 0 part sdd 8:48 0 2.7T 0 disk └─sdd1 8:49 0 2.7T 0 part └─md0 9:0 0 8.2T 0 raid5 └─md0p1 259:0 0 8.2T 0 md /srv/media sde 8:64 0 2.7T 0 disk └─sde1 8:65 0 2.7T 0 part └─md0 9:0 0 8.2T 0 raid5 └─md0p1 259:0 0 8.2T 0 md /srv/media sr0 11:0 1 7.3G 0 rom So, it looks like the partition on sdc isn't being seen as a raid partition anymore. FWIW; root at debian:/var/log# cat /proc/mdstat Personalities : [raid6] [raid5] [raid4] md0 : active raid5 sdb1[0] sde1[4] sdd1[2] 8790405120 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/3] [U_UU] bitmap: 4/22 pages [16KB], 65536KB chunk unused devices: <none> So, what's my next step? Looks to me like I need to recreate sdc1 as a raid partition, then regrow the array. Is this correct? In the meantime, I'm reading what I can and copying data off the array. Nothing critical, but it'd be annoying to lose. Let me know if I left out required information. Thanks. Mark