@Stephan, As I understand this, you are using an md raid1 to save power
and have a "backup"?  This is failure prone and dangerous to your data.
The MD layer is pretty resilient but it is meant for RAID of like disks.
This is why the numbers are weird.  It "expects" a scsi/sata hdd and
works best if they are matched, i.e. same disk model.  Mixing in a usb
stick is not the same thing.  You save little, if any, power because an
array restore requires a complete disk copy, not an update of some
number of out-of-date blocks.  I wouldn't be surprised if it consumes
even more since it is a steady state transfer load until the restore is
complete.  This restore, depending on subsystem traffic and disk size
can take a significant amount of time.  It can also leave your system
vulnerable.  I have heard of reiser filesystems fail w/ compromised raid
arrays, which this is in your powersaving mode.  There is power
management in the newer kernels to cycle down the hdd to conserve power
but this takes careful tuning.  If you want backup with this mix of
devices, use rsync to the usb stick.  This will be consistent and only
write what is needed.  A usb stick is not the same as an SSD.  The
former is meant as a replacement for floppies, namely lower than hdd
transaction rates, removability, and expected limits to lifetime.  The
later is meant for hdd type applications.  I suggest you reconsider your
configuration.

I did not "invalid" this bug yet pending your reply.

-- 
Raid1 HDD and SD card -> data corruption (bio too big device md0 (248 > 200))
https://bugs.launchpad.net/bugs/320638
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