Why do I get a DegradedArray event with mdadm

• does a raid5 array always start in degraded mode at first ?

Yes. At least, it always has for me. It makes sense; for any one of the disks to be able to fail, the disks must be synced. This will involve writing an equivalent of one whole disk's worth of data, which will take a while. Until this has been completed, the array is seen as "degraded".

Update: From the mdadm manual:

When creating a RAID5 array, mdadm will automatically create a degraded array with an extra spare drive. This is because building the spare into a degraded array is in general faster than resyncing the parity on a non-degraded, but not clean, array. This feature can be overridden with the --force option.