I’ve recently had the opportunity to set up a Promise DS4600 connected to a Mac mini Server. Using the supplied admin utility, it’s not immediately obvious how to set it up as a 3 disk RAID 5 and use the 4th disk for a hotspare. In fact, the GUI gives no indication whatsoever that it’s even possible.
4TB is quite a huge amount of storage for the kinds of environments in which these units will be used, even 3TB with the protection of RAID, is quite a lot, considering that RAID ? Backup. 2TB is still a decent amount of storage, which is what you get from a 3x 1TB RAID5 array and you then have the safety net of a hotspare disk. So, how do you do it?
Thinking laterally, it’s actually quite easy. Pop one of the disks out, ignore the beeping and then use the admin utility to configure it as a 3 disk RAID 5 volume. Once it’s initialised the array, and created the partition, insert the 4th disk and erase it. Although it doesn’t say so, this disk is now a hotspare and the array will rebuild onto this disk should one of the other three fail.