This Problem was reported by a customer and I was able to reproduce.

"When postConfigure creates a list of dbroots and assigns them device nodes, it starts at 'g' and increments, but it always skips 'k'.
When we tried to start the cluster with 5 PMs it failed to start complaining about the volume not found on pm5"

use postConfigure to create a 5 pm system with 1 dbroot per pm.

from pm1:

[mariadb-user@ip-172-30-0-43 ~]$ cat /etc/fstab
LABEL=root / xfs defaults,relatime 1 1
tmpfs /dev/shm tmpfs defaults 0 0
devpts /dev/pts devpts gid=5,mode=620 0 0
sysfs /sys sysfs defaults 0 0
proc /proc proc defaults 0 0
/root/.myswapfile swap swap defaults 0 0
ephemeral0 /media/ephemeral0 auto defaults,nofail,comment=cloudconfig 0 2
ephemeral1 /media/ephemeral1 auto defaults,nofail,comment=cloudconfig 0 2
swap none swap sw,comment=cloudconfig 0 0
/dev/xvdg /home/mariadb-user/mariadb/columnstore/data1 ext2 noatime,nodiratime,noauto,user 0 0
/dev/xvdh /home/mariadb-user/mariadb/columnstore/data2 ext2 noatime,nodiratime,noauto,user 0 0
/dev/xvdi /home/mariadb-user/mariadb/columnstore/data3 ext2 noatime,nodiratime,noauto,user 0 0
/dev/xvdj /home/mariadb-user/mariadb/columnstore/data4 ext2 noatime,nodiratime,noauto,user 0 0
/dev/xvdl /home/mariadb-user/mariadb/columnstore/data5 ext2 noatime,nodiratime,noauto,user 0 0
[mariadb-user@ip-172-30-0-43 ~]$

mcsadmin> getst
getstorageconfig Mon Jul 31 14:29:50 2017

System Storage Configuration

Performance Module (DBRoot) Storage Type = external
User Module Storage Type = internal
System Assigned DBRoot Count = 5
DBRoot IDs assigned to 'pm1' = 1
DBRoot IDs assigned to 'pm2' = 2
DBRoot IDs assigned to 'pm3' = 3
DBRoot IDs assigned to 'pm4' = 4
DBRoot IDs assigned to 'pm5' = 5

Amazon EC2 Volume Name/Device Name/Amazon Device Name for DBRoot1: vol-079b3cbedef7f5e63, /dev/sdg, /dev/xvdg
Amazon EC2 Volume Name/Device Name/Amazon Device Name for DBRoot2: vol-024fe24c31bdc84fd, /dev/sdh, /dev/xvdh
Amazon EC2 Volume Name/Device Name/Amazon Device Name for DBRoot3: vol-044553de1a940d7db, /dev/sdi, /dev/xvdi
Amazon EC2 Volume Name/Device Name/Amazon Device Name for DBRoot4: vol-0dd0e0288b7ec1ed0, /dev/sdj, /dev/xvdj
Amazon EC2 Volume Name/Device Name/Amazon Device Name for DBRoot5: vol-05dd275951bb6163f, /dev/sdl, /dev/xvdl

----- Performing Install on 'pm5 / i-0d89929eacf3c73a1' -----

Install log file is located here: /tmp/pm5_binary_install.log

MariaDB ColumnStore Package being installed, please wait ...
Failure with a remote module install, check install log files in /tmp