mdadm partiton not large enough to join array

Question

I've used Linux on and off for the last many years, mainly as dual boot or as Virtual Machines, but when I decided to build a Dev Laptop I though I'd bite the bullet and put Linux on as the base OS.

In the past I've had issues so thought I'd use RAID to give some resilience, after a bit of head scratching I found the following article [URL="https://wiki.debian.org/Multi%20HDD/SSD%20Partition%20Scheme"]Multi HDD/SSD Partition Scheme[/URL].

In summary it advises building 'nested' arrays so you end up with something like the following

md6 - /dev/sda1
    - /dev/md60

md60 -/dev/sdb1
     -/dev/sdc1

I tried that using an Ubuntu LiveCD, and managed to build the arrays fine, however installing to them was a nightmare, and I never managed to get it working (failed during install). I then moved on to try Debian, and although it installed I couldn't get the drivers for my graphics card to work.

So onto the third distro (suse) and although it got father but appeared to fail when it tried to assemble the RAID. So I though I'd simplify it by only creating the top level RAID (md6) with the one partition and with one device (so it didn't think it was degraded).

So I now have a fully working Suse install!!!

However I don't have a RAID. so I tried to assemble the raid,

mdadm /dev/md6 --add --write-mostly /dev/md60

or (as some one said it worked this way...

mdadm --add /dev/md6 --write-mostly /dev/md60

also tried with and without --write-mostly but always got

mdadm: /dev/md60 not large enough to join array

so a quick look at mdadm --detail gave the following...

/dev/md6:
        Version : 1.2
  Creation Time : Sun Apr 27 14:42:13 2014
     Raid Level : raid1
     Array Size : 204341248 (194.88 GiB 209.25 GB)
  Used Dev Size : 204341248 (194.88 GiB 209.25 GB)
   Raid Devices : 1
  Total Devices : 1
    Persistence : Superblock is persistent

    Update Time : Fri May  9 20:38:51 2014
          State : clean 
 Active Devices : 1
Working Devices : 1
 Failed Devices : 0
  Spare Devices : 0

           Name : linux:6
           UUID : b24522f2:8622c47b:be61720b:e69b9dfd
         Events : 23

    Number   Major   Minor   RaidDevice State
       0       8       35        0      active sync   /dev/sdc3

and

/dev/md60:
        Version : 1.2
  Creation Time : Fri May  9 20:21:00 2014
     Raid Level : raid1
     Array Size : 204343296 (194.88 GiB 209.25 GB)
  Used Dev Size : 204343296 (194.88 GiB 209.25 GB)
   Raid Devices : 1
  Total Devices : 1
    Persistence : Superblock is persistent

  Intent Bitmap : Internal

    Update Time : Fri May  9 20:35:21 2014
          State : active 
 Active Devices : 1
Working Devices : 1
 Failed Devices : 0
  Spare Devices : 0

           Name : hotBlack.ons:60  (local to host hotBlack.ons)
           UUID : 39249c62:f41412c9:b1149c7d:fe4222a0
         Events : 2

    Number   Major   Minor   RaidDevice State
       0       8        7        0      active sync   /dev/sda7

so that looks like

md6  Array Size : 204341248 (194.88 GiB 209.25 GB)
md60 Array Size : 204343296 (194.88 GiB 209.25 GB)

so that looks like md60 is 2MB bigger than md6 (which is how I set it up), so how can it be too small?

What am I missing here?

Any advice gratefully received?

Nick(c)