1. When stuff is written to a RAID controller, the RAID controller caches/holds that data then reports to the application that it is done so it can move onto the next task, then it writes everything all at once to the disks later. This cached data is stored in RAM memory, so in order to protect the integrity of the data on the disks in the event of a power outage, etc., a battery is required to power the RAM module to keep the data alive until the server is powered back on, when the writes can then be committed to the disks. Typically when the battery suffers failure, the controller disables write caching to protect data integrity. This causes a performance degradation without the caching. 2. This would not normally corrupt data. By design, it should not, but many things can happen, so you should never be without a backup, and this is also why you shouldn't do this for "fun". A RAID 5 can withstand a single drive failure. Performance is significantly degraded, but the data is still up and alive. Reinserting the disk will typically result in a rebuild - the controller rebuilds the disk to match the data in the RAID 5 array. RAID 0 does NOT stay online in such a case - if you remove a drive in a RAID 0, then you would possibly have destroyed the data. 3. Removing the drive and reinserting it while the system is off will not result in anything. It would come back on normally. 4. No, never power down a server to replace a drive that can be inserted from the front of the machine. Simply remove the failed drive, insert the replacement. It may start rebuilding automatically. If not, then you need to assign it as a hot-spare to start the rebuild. And if the drive has a configuration on it already, it will show as foreign before you can do anything with it, in THIS CASE ONLY, you would clear the foreign configuration before assigning it as a hot-spare. You can use the CTRL-R utility to do this, but I would recommend doing it from OpenManage Server Administrator from the OS. 
Hi everyone , Please allow me to ask about some points here , and i am sorry if it was asked before. but i need an exact explanation for my questions if you can please ,  1- what is the role of the RAID Battery in the server?!!! and what happened if it was bad or faulty ? 2-on a server have a 7 Drives "as example" with Raid 5 Configuration ,and while the server is working and UP what happened if i removed one of the SAS Hard disk Drives out of the server?!! and what happened if i attatched it again after a while ?? does this will corrupt the RAID Array or make data loss or the server will stop working?! 3-on a server have a 7 Drives "as example" with Raid 5 Configuration , and while the server is OFF and Shutted Down , what will happened if i remove one of the SAS Hard Disk Drives and returne it Back ?!! does this will corrupt the RAID array and make data loss ??!! 4-in case of Dell Poweredge servers like the R730 Model, if i have a RAID arary on a working server and one of the drives failed , how can i replace this faulty drive and rebuild the RAID array again without data loss? ; do i will need to shut down the server to rebuild the RAID or it will reBuilt it self Automatically ?!! also do i need to go to Life cycle controller to modify or reconfigure anything to rebuild the array??!!  sorry for the long questions , but i appreciated your answers and efforts here at Dell Community. Thanks
RAID Configuration Status