Single-drive RAID0

Neil,

I try to create a single-drive RAID0 array with data preservation.
It means I have a single disk with partitions and data on them. 
I would like to migrate this single disk to RAID0 array and then grow it to N devices.
Of course I expect partitions with my useful data will be mapped to corresponding MD block devices (/dev/md126p1, /dev/md126p2 etc.). Indeed they are but when I try to mount new block device it disappears from /dev directory and mount is not succeeded. 
/var/log/messages shows all the time: 
md126: p1
md126: detected capacity change from 0 to xxxxxxxxxxxxx

It is not always reproducible but very often.
I try this operation on 0.9 and imsm metadata. Symptoms are similar.

Do you have any suggestions? How to fix this issue?

Regards
Krzysztof