Mac OS Server Flashing Folder

[PeterH]

OK, we have a Mac OS X Server about 5 years old. Been working fine, but this morning found it hung on the login screen with the spinning coloured wheel. Forced it to shut down, but now just get a flashing folder with a question mark. Seems like it can't find its startup disk, so booted off the server installation DVD and went into Disk Utility - it can only see the DVD drive. Went into RAID utility - everything looks good - 3 good HDDs, a good RAID Set called RS1 and a volume called R1V1 - everything seems fine. But nothing else can see the disk. Startup utility can't see it, Disk utility can't see it, the OS X Server Installer can't see it. For some reason it's like the RAID isn't presenting the disk to the host? Have tried an NVRAM reset, but no difference. Any ideas anyone?

Many thanks,

PH