Today I took the internal sd out from GLO， found it is Sandisk, class4, the capacity is 4G.
As I knew, GLO's space shoud be 2G, so I watched it's partition via LinuxReader, found 1.9g is unused. Then I tried to expand the reader partition to cover this freespace and got sucess.
Now when I link my GLO to PC, I have about 3G space to put book.
I think maybe sometime 2GB microsds were cosumed up in KOBO‘s warehouse， so they put a 4GB microsd.