it’s technically possible
1. you will get RAID1 with total capacity of smaller of the HDD = 1TB
2. your disk group performance will be affected by slower of them
3. two diff disk drives with diff firmware to RAID1 is a call for troubles
then you have few options for capacity expansion:
- RAID0 and you will get 5TB in total. But you need take into consideration risk of data lose. Then a backup is your friend.
- two same HDDs into RAID1
- expansion box DX517 with next 5bays. But don’t make single large disk group across all the 7 bays. two independent is better.
If disk1 (the working disk) is 4TB (SHR), you cannot add a 1TB drive to that volume. You could only create a second volume containing the lone 1TB drive. You would need another 4TB drive (or larger) to combine with the existing SHR volume.