It may seem like a good idea to slowly replace the hard disk drives in a RAID array for faster versions, but this may create some unwelcome side effects. Also when planning a new RAID array and you already have some drives which you want to use it is best to buy direct matches or buy a complete set of new disks.
Many RAID arrays are set up to detect when a drive is performing slower than others contained in the array, at which point they may be considered to have failed, even when still functioning without any errors. Mixing drives manufactured for their efficiency or SMR drives with helium and enterprise level storage are extreme examples of hard disk mismatches which can cause problems.
With storage solutions expected to operate continually for over increasingly extended periods of time, this is becoming more of an issue when any drive fails and needs to be replaced. It is natural not to want to waste any drives which are still working correctly, but incorporating them into a RAID array may be a bad decision if the new drives are not well matched.
Even more extreme would be to mix SSD technology with traditional spinning hard disk drives, which could very quickly cause the latter drives to be considered as failing, even if they are still working within design specifications. Even mixing brands or different models of drives may also not be handled well by the RAID controller.
