RAID 5 - defrag?

You can defrag a Windows "disk" that sits on a hardware RAID 5 just as well as a non-RAID drive.  I would first run a Consistency Check on the Virtual Disk/RAID array, then run your defrag and chkdsk.

Spacore

As far a safety of running a defrag, never had an issue in 20 years due to defragging a raid array.

Using "Diskeeper" or "Perfectdisk" on all my servers, running a scheduled off hours defrag nightly, I have a consistent 4-6% improvement over raids which are infrequently defragged ( say weekly). Also these programs, as others, can perform "boot time" defrags of program files which can not be defragged normally, as they run resident ( ega  SQL, backup programs, AV programs etc), which gives roughly a 10-15% increase to the programs mentioned and adds to overall raid speed). I run boot time defrags monthly generally, but I am primary dealing with small business ( under 200 employees, mostly under 50).

http://www.diskeeper.com/

http://perfectdisk.raxco.com/

As to consistency checks, I run them via scheduled tasks, once a week. I run Patrol Reads manually, twice a week, scheduled via taskmanager. Patrols Reads is the most critical to run as often as practical especially in arrays with a large number of disks or high capacity disks. Running Patrol Reads becomes more  critical as array size increases. Default for Patrol Reads is to run auto, but I do not want it running during normal production hours. Unlike a consistency check, PRs check the entire surfaces of the involved disks in an array, CCs only check the data area. Most arrays die in a rebuild due to multiple errors found  in areas of the array in which data does not occupy.

Thanx for the info!

Ran a defrag, and now users are complaining that performance is MUCH worse.  I'm wondering about a drive I put into the server.  I believe it's bad.  Can I remove a drive from a RAID-5 while it's running with no issues?  Should I stop it first?

A bad drive can absolutely corrupt your data.  Did you perform a Consistency Check first?  This would have reduced (not necessarily eliminated) the amount of damage caused from bad logical blocks on the array.  You can remove a drive from a RAID 5 without it going offline - and yes, you should stop it first (Prepare to Remove or Force Offline) - but that may not help your situation ... the damage may already be done.

Dilligently check to make sure your cache is working.   It can get shut off after failures or certain raid operations and need to be manually set back to on.

Like the Flash mentioned could be a bad drive, also a different firmware on the drive could be an issue. You might try running chkdsk to see the amount of file damage, do not run it with with the /F switch as this tries to correct issue if they are found, with the /F switch it may cause more damage; just plain chkdsk just points out file problems (read only). Sounds like the Dell disk diagnostic is in order.