Unwanted and un-modifiable behaviour of Smart Array "Mixed mode". Drive letter for OS system drive (Logical drive created by RAID setup) will change when new unassigned drives are inserted into the system.
It seems to be related to the "Mixed mode" behaviour. Unfortunately, according to the "HPE Smart Array SR Gen10 Configuration Guide" (part no. 882375-002), the mixed mode of Port settings is "cannot be disabled".
*** Power up with RAID 0 on hdd 1
[ict@server002 ~]$ ls -al /dev/sd*
brw-rw----. 1 root disk 8, 0 May 2115:52 /dev/sda
brw-rw----. 1 root disk 8, 1 May 2115:52 /dev/sda1
brw-rw----. 1 root disk 8, 2 May 2115:52 /dev/sda2
brw-rw----. 1 root disk 8, 3 May 2115:52 /dev/sda3
brw-rw----. 1 root disk 8, 16 May 2115:52 /dev/sdb
[ict@server002 ~]$ echo "Insert hdd 2"
Insert hdd 2
[ict@server002 ~]$ ls -al /dev/sd*
brw-rw----. 1 root disk 8, 0 May 2115:52 /dev/sda
brw-rw----. 1 root disk 8, 1 May 2115:52 /dev/sda1
brw-rw----. 1 root disk 8, 2 May 2115:52 /dev/sda2
brw-rw----. 1 root disk 8, 3 May 2115:52 /dev/sda3
brw-rw----. 1 root disk 8, 16 May 2115:52 /dev/sdb
brw-rw----. 1 root disk 8, 32 May 2116:49 /dev/sdc
[ict@server002 ~]$ echo "Insert hdd 3"
Insert hdd 3
[ict@server002 ~]$ ls -al /dev/sd*
brw-rw----. 1 root disk 8, 0 May 2115:52 /dev/sda
brw-rw----. 1 root disk 8, 1 May 2115:52 /dev/sda1
brw-rw----. 1 root disk 8, 2 May 2115:52 /dev/sda2
brw-rw----. 1 root disk 8, 3 May 2115:52 /dev/sda3
brw-rw----. 1 root disk 8, 16 May 2115:52 /dev/sdb
brw-rw----. 1 root disk 8, 32 May 2116:49 /dev/sdc
brw-rw----. 1 root disk 8, 48 May 2116:56 /dev/sdd
[ict@server002 ~]$ echo "Insert hdd 4"
Insert hdd 4
[ict@server002 ~]$ ls -al /dev/sd*
brw-rw----. 1 root disk 8, 0 May 2115:52 /dev/sda
brw-rw----. 1 root disk 8, 1 May 2115:52 /dev/sda1
brw-rw----. 1 root disk 8, 2 May 2115:52 /dev/sda2
brw-rw----. 1 root disk 8, 3 May 2115:52 /dev/sda3
brw-rw----. 1 root disk 8, 16 May 2115:52 /dev/sdb
brw-rw----. 1 root disk 8, 32 May 2116:49 /dev/sdc
brw-rw----. 1 root disk 8, 48 May 2116:56 /dev/sdd
brw-rw----. 1 root disk 8, 64 May 2116:56 /dev/sde
[ict@server002 ~]$
*** Power off
2018-May-23 Power up with all drive inserted already
[ict@server002 ~]$ ls -al /dev/sd*
brw-rw----. 1 root disk 8, 0 May 2310:43 /dev/sda
brw-rw----. 1 root disk 8, 16 May 2310:43 /dev/sdb
brw-rw----. 1 root disk 8, 32 May 2310:43 /dev/sdc
brw-rw----. 1 root disk 8, 48 May 2310:43 /dev/sdd
brw-rw----. 1 root disk 8, 49 May 2310:43 /dev/sdd1
brw-rw----. 1 root disk 8, 50 May 2310:43 /dev/sdd2
brw-rw----. 1 root disk 8, 51 May 2310:43 /dev/sdd3
brw-rw----. 1 root disk 8, 64 May 2310:43 /dev/sde
[ict@server002 ~]$
*** Power off
2018-May-24 Power up with 4th drive unpluged
[ict@server002 ~]$ ls -al /dev/sd*
brw-rw----. 1 root disk 8, 0 May 2408:37 /dev/sda
brw-rw----. 1 root disk 8, 16 May 2408:37 /dev/sdb
brw-rw----. 1 root disk 8, 32 May 2408:37 /dev/sdc
brw-rw----. 1 root disk 8, 33 May 2408:37 /dev/sdc1
brw-rw----. 1 root disk 8, 34 May 2408:37 /dev/sdc2
brw-rw----. 1 root disk 8, 35 May 2408:37 /dev/sdc3
brw-rw----. 1 root disk 8, 48 May 2408:37 /dev/sdd