I have the same problem with 2 HHD 30 Go (exactly the same), with raid-1 installed, the available space disk is only 15 Go!
Here are more informations :
with fdisk /dev/hda (command p)
hda 255 heads, 63 sectors, 3649 cyl.
units = cyl. of 16065 * 512 bytes
device Boot start end blocks Id System
/dev/hda1 * 1 33 265041 fd Linux raid autodetect
/dev/hda2 34 3649 29045520 5 Extended
/dev/hda5 34 35 16033+ fd Linux raid autodetect
/dev/hda6 36 1843 14522728+ fd Linux raid autodetect
with fdisk /dev/hdb (command p)
hdb 16 heads, 63 sectors, 58168 cyl.
units = cyl. of 1008 * 512 bytes
device Boot start end blocks Id System
/dev/hdb1 * 1 521 262552+ fd Linux raid autodetect
/dev/hdb2 522 58168 29054088 5 Extended
/dev/hdb5 522 542 10552+ fd Linux raid autodetect
/dev/hdb6 543 29356 14522224+ fd Linux raid autodetect