SOLVED
You haven't mentioned how you are using this array like Virtual or Liner.
I am assuming you are using this array as Liner then the concept that you are using is Virtual Disk (Vdisk).
I am assuming your existing datastore means some MSA volume created from some MSA Vdisk which is made with 7 SAS 600GB drives. Now you want to expand it with two 900GB drives. Yes it's possible but personally I will never recommend. I prefer to take backup of data, delete the vdisk, replace all 600GB drives with 900GB drives. Re-create new Vdisk and volumes. Then restore data from backup. This approach more fast than vdisk expansion because it may take days or weeks or months to complete. Also data risk because if any issue in between vdisk expansion then you can't stop it, only option will be to delete the vdisk.
To answer all your queries,
1. Add 2 900GB HDD in empty slots and add them to the datastore
Ans: You need to first add 2 900GB drives to the existing Vdisk. Once Vdisk expansion completed then you can increase the size of the datastore or the MSA volume.
2. Then one by one replace the 600 GB with 900 GB disks. (After each disk wait for the rebuild of the RAID).
Ans: Yes you can do the same. Just keep in mind that smallest size drive which is 600GB here determines the logical capacity of all other drives in this vdisk. So it doesn't matter how many 900GB drives available.
3. When all disks are replaced grow the RAID to the new size.
Ans: Yes possible
Hope this helps!
Regards
Subhajit
I am an HPE employee
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