I added a new hard drive to my existing cauldron system, and found that while I could delete and create partitions on it, I could not create an ext4 filesystem.  Formatting either with diskdrake or directly with mkfs.ext4 got the error:

/dev/sdc1 is apparently in use by the system; will not make a filesystem here!

Googling this turned up a lot of useless stuff blaming this on leftover RAID signatures, but I finally found the diagnosis in a RedHat post.  It appears that MGA makes any newly-detected disk eligible for multipath, and *that* constitutes being "in use by the system".

[root@ftgme2 dev]# multipath -ll               
Hitachi_HUA723030ALA640_MK0373YHJU4VVC dm-1 ATA,Hitachi HUA72303
size=2.7T features='0' hwhandler='0' wp=rw
`-+- policy='service-time 0' prio=1 status=active
  `- 3:0:0:0 sdc 8:32 active ready running

The RedHat suggestion was to put the following into /etc/multipath.conf (we ship an empty file):

blacklist {
        devnode "^sd[a-z]"
}

which prevents multipath processing for sd* disks.  Doing this followed by

[root@ftgme2 dev]# systemctl restart multipathd
[root@ftgme2 dev]# multipath -ll
[root@ftgme2 dev]#

worked, and I was able to format sdc1.

I am totally unfamiliar with RAID and multipath, so I don't know what the recommendation should be for MGA, but the current state of affairs is going to drive people nuts.  Nobody who adds a disk to a system is going to expect formaat to fail with a cryptic message, and it took me a day and a half of researching to find this diagnosis.

We should either ship the above blacklist or else improve on it.
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