OK, more info: # mkraid /dev/md0 handling MD device /dev/md0 analyzing super-block disk 0: /dev/sdb1, 4257193kB, raid superblock at 4257088kB disk 1: /dev/sdc1, 4257193kB, raid superblock at 4257088kB disk 2: /dev/sdd1, 4192933kB, raid superblock at 4192832kB /dev/md0: Invalid argument leaves this in the logs: Mar 22 16:32:28 raidtest kernel: bind<sdb1,1> Mar 22 16:32:28 raidtest kernel: no chunksize? Mar 22 16:32:28 raidtest kernel: bind<sdc1,2> Mar 22 16:32:28 raidtest kernel: no chunksize? Mar 22 16:32:28 raidtest kernel: bind<sdd1,3> Mar 22 16:32:28 raidtest kernel: no chunksize? Mar 22 16:32:28 raidtest kernel: sdd1's event counter: 00000000 Mar 22 16:32:28 raidtest kernel: sdc1's event counter: 00000000 Mar 22 16:32:28 raidtest kernel: sdb1's event counter: 00000000 Mar 22 16:32:28 raidtest kernel: md: bug in file md.c, line 1536 Now, md.c is one of the things modified by the patch, so I looked at it a bit: * chunk-size has to be a power of 2 and multiples of PAGE_SIZE */ if ( (1 << ffz(~chunk_size)) != chunk_size) { MD_BUG(); return -EINVAL; and figured, contrary to documentation, that it actually needs a chunk size defined. (http://www.ostenfeld.dk/~jakob/Software-RAID.HOWTO/Software-RAID.HOWTO-3.html#ss3.2, which I thought was the latest info, does not include chunk_size in the example for a linear device.) Adding a chunk_size to /etc/raidtab fixed my problem. Having the persistent-superblock defined lets it be seen on boot-up, which is nice! I can just stick an entry for /dev/md0 in /etc/fstab. Thanks to all who helped! -Ross