[BadKosh]

I have a late 2008 Mac Pro 2.66 with the older RAID card and four 750 GB drives.
It looks like the user had RAIDed the drives and then decided not to.
All 4 drives work as normal drives but show up in the RAID Utility as 4 drives, a RAID set!

can I delete the RAID set without ruining whats on the drives?
The system drive boots the system when the other 3 drives are removed,
and the RAID Utility shows just the one drive and one drive in the RAID set.

The RAID card needs to come out so a Fiber-channel card can be installed into slot 4.

Can the Mac Pro even operate without the card?

[OreoCookie]

If you delete the RAID set, you delete the data. You may be able to recover the data again, but I removing the drives from the RAID is like repartitioning a harddrive.

If you want to un-RAID the drives, you need to copy the data someplace else first.

[reader50]

You could try using a partition utility to delete Apple's "Booter" partition from the partition maps. The presence of this partition may be what utilities are responding to.

This ought to be safe enough - though I'd try it on some USB flash drives first, just to be sure.