I had an interesting (only because I do backups) issue with Windows 2003 Server.

I wanted to replace two 40-GB drives that hold my data (the OS is also mirrored on two other disks), that are RAIDed through Windows 2003's mirroring feature, with two 160-GB drives. The server has 4 IDE channels. I added the 160GB drive to a separate channel - the one used by the CD-ROM, just so I don't fiddle with the data drives.

After rebooting, the system hung until I disconnected one of the 40GB data drives. Perhaps it coincidentally failed. Anyway, when the system came up, the remaining mirrored drive reported itself as "unallocated" space - no data. This was the type of problem RAID was supposed to avoid. Fortunately, I had backed up the data accross the network to a separate PC, so nothing was lost. If I had not, it would have been a disaster.

I think I'll abandon Windows' mirroring and look for a hardware solution.