I'll try to help, having used raid5 in lmce in the past (ditched it when moving to 810). Now I instead use an external USB disk as backup for my data and non-raid disk in lmce. As RAID is no backup solution, you need backup anyway. RAID is really best for increasing availability of the data.
1. Did you format using ext3? In that case it may be that you see the same problem as mentioned here http://boncey.org/2006_11_18_reclaiming_ext3_disk_space
, that disk space is reserved for the super-user.
2. Don't know for sure what lmce does when it sets up a raid5, assuming you set it up in the web admin. I would try to verify that the raid settings are in fact correct, and check from the console if you see anything on the drives.
You say upgrade, and I assume you did a new install with 810 (as there is no upgrade path). Did you keep the install from messing with your raid disks?