How to append data in a file by dd?

dd if=/dev/shm/test of=/data/sdb/test bs=1G oflag=append conv=notrunc 

That is what I think you should have used.

REF : https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=373736

What about:

 dd if=/dev/shm/test bs=1G >>/data/sdb/test