[CentOS] mdraid doesn't allow creation: device or resource busy

Dear fellow CentOS users,

I have never experienced this problem with hard disk management before 
and
cannot explain it to myself on any rational basis.

The setup:
I have a workstation for testing, running latest CentOS 7.3 AMD64. I am
evaluating oVirt and a storage-ha as part of my bachelors thesis.
I have already been running a RAID1 (mdraid, lvm2) for the system and
some oVirt 4.1 testing. Now I added 6x 500 GB platters from an old
server running Debian 8 Jessie with software RAID of a similar fashion
as well. That would unexpectably prevent the system from booting
past something like (I copied it from the working setup), I ran it over
night, so it was actually about 16 hours:
"A start job is running for dev-mapper-vg0\x2droot.device (13s / 1min
30s)"

Can it be just some kind of a scan, which takes so long? The current 
throughput based
on time (16 h) and capacity (3 TB) would be about 50 MBps. (Those drives 
can be pretty
slow when writing, dd showed about 30 MBps, the write cache is off.)

This is actually repeatable. If I unplug those drives and boot again, it 
all works.

I don't know, if it helps but before that I had two screens full of:
"dracut-initqueue[331]: Warning: dracut-initqueue timeout - starting
timeout scripts"

Well, I proceded without this array, and after it booted I connected the
array of 6 hard disks again. They were recognized etc. The problem is, I
cannot do much. I can dd from and to the harddrives, I can create and
delete partitions but I cannot create an md raid array out of them, I
cannot create a physical volume or format them with a filesystem. I even
tried overwriting all of those harddrives with zeroes, which worked but
didn't help at all with the creation of the array afterwards.

mdadm --create /dev/md6 --level=5 --raid-devices=6 /dev/sd[cdefgh]1

"mdadm: cannot open /dev/sdc1: Device or resource busy"

with pvcreate, it seems as if there was no device, but I clearly see it
in /dev/sdc1...

"Device /dev/sdc1 not found (or ignored by filtering)."

partprobe yields:

"device-mapper: remove ioctl on ST3500630NS_9QG0P0PY1 failed: Device or
resource busy
Warning: parted was unable to re-read the partition table on
/dev/mapper/ST3500630NS_9QG0P0PY (Device or resource busy).  This means
Linux won't know anything about the modifications you made.
device-mapper: create ioctl on ST3500630NS_9QG0P0PY1part1-mpath-
ST3500630NS_9QG0P0PY failed: Device or resource busy
device-mapper: remove ioctl on ST3500630NS_9QG0P0PY1 failed: Device or
resource busy"

In some forums, it was suggested, dmraid (yes, the old) could be the
trouble. I eliminated this hypothesis (overwritten all with zeroes,
fakeraid was never present with these disks). Also multipathd/
dm_multipath could be the trouble, someone suggested. The problem is, if
I was to remove device-mapper-multipath, I would loose oVirt-engine,
because it has multipath as dependency for some reason.

Do you have any ideas? What logs/ information should I provide if you
want to have a look into this.

Best regards
Adam Kalisz