Partitioned mariadb table files(the .ibd ones) placed outside of data directory return back to data directory itself after altering the table in use While Partitioning large tables in Mariadb we want some partitions to be placed in another storage device with different speed and capacity. That is our requirement. 
For that we are trying to perform some testing in our local set up,where we have placed some partitions of a table(Actually partitions of the innodb tables files, the .ibd ones) outside of the data directory by means of alter table statements with data directory clause. 
But the problem is that whenever we alter the table after partitioning ,the partitioned .ibd files placed outside of datadir comes back to the datadir which is really unexpected. 
So can anyone please help in this context to clarify whether this is a issue with mariadb or  with the concept itself described as above.
That's a problem with ALTER.  File a bug report if you like.

I consider RAID striping to be better than manually placing pieces of the database across physical volumes.

What kind of ALTER are you doing?  Adding an index is different than adding a column is different than fiddling with the partition structure.  There may be a workaround.