Question RE: PV 725N RAID configuration/controller

I've recently take over the management of a PV 725N w/ a 4 drive IDE RAID configuration.  It appears to have the standard volume setup with 2 mirrored OS drives and one RAID 5 large data drive.  The interesting thing, however, is twice within the last month a single drive has gone offline and taken the entire RAID volume with it.  To correct this I need to reactivat the drive and then do the same to the volume.  The volume doesn't come back on line until its completely rebuilt and then the entire system is restarted.  

My first issue is there's nothing in the logs to indicate why the drive dropped offline to begin with but I assume its having some difficulties so I have new drives on order. 

Second and more importantly, any Dell RAID that I'm familiar with, mostly SCSI RAID, doesn't drop the entire volume with the loss of just one disk.  In previous experience with other RAIDs the volume stays up and I'm able to simply pull the bad disk and replace it.  After which a rebuild starts automatically with no interruption to the users.  Is there some limitation to the controller in the PV 725N that doesn't provide for this or do I have larger problems? 

Any help is appreciated. 

Thanks
Rob