From your description it looks like when YOU plugged in an additional device to the system via the external adapters, this effected the device ordering… and OMV got confused.
Removing the device made OMV happy.
As for the slow performance, something may still be out of whack.
Remember when dealing with drives, never refer to the device as /dev/sd[abcde], as devices can (and will) be mounted differently on the next boot, depending on how long it takes for devices to startup, and scan order.
Usually /dev/disk/by-id, or /dev/disk/by-uuid/ works much better.
On my system:
pinas:/dev/disk/by-id $ ls -al
total 0
drwxr-xr-x 2 root root 200 May 18 11:18 .
drwxr-xr-x 7 root root 140 May 18 11:18 ..
lrwxrwxrwx 1 root root 9 May 18 11:18 ata-MICRON_M510DC_MTFDDAK960MBP_15170FAB1DD5 -> ../../sdd
lrwxrwxrwx 1 root root 9 May 18 11:18 ata-MICRON_M510DC_MTFDDAK960MBP_153511F380DE -> ../../sdb
lrwxrwxrwx 1 root root 9 May 18 11:18 usb-JMicron_Generic_0123456789ABCDEF-0:0 -> ../../sda
lrwxrwxrwx 1 root root 10 May 18 11:18 usb-JMicron_Generic_0123456789ABCDEF-0:0-part1 -> ../../sda1
lrwxrwxrwx 1 root root 10 May 18 11:18 usb-JMicron_Generic_0123456789ABCDEF-0:0-part2 -> ../../sda2
lrwxrwxrwx 1 root root 9 May 18 11:18 usb-Pinas_SATA_0000000000EC-0:0 -> ../../sdc
lrwxrwxrwx 1 root root 9 May 18 11:18 wwn-0x500a07510fab1dd5 -> ../../sdd
lrwxrwxrwx 1 root root 9 May 18 11:18 wwn-0x500a075111f380de -> ../../sdb
As you can see;
/dev/disk/by-id/wwn-0x500a075111f380de is currently mounted as /dev/sdb
So… instead of /dev/sdb, I should use /dev/disk/by-id/wwn-0x500a075111f380de