Should I upgrade 2 - 2TB drives at a time in v2?

I have 2 Drobo units. I want to add drives to the v2 and then put the two 1TB drives into the v1 unit.

V1 with 2-1TB & 2-500GB - 1.48 used and 334.96 free

v2 with 4-1TB drives - 2.2TB used 506 Free
Setup as (Drive-H, 2TB with 1.41 used) and (Drive I, 2-TB with 831GB used)

I have an unused Seagate 1.5TB with CC1H (3rd one from Seagate, afraid to put into the unit)

I want to increase the capacity of v2 as I have another TB to save.

Should I get 2-2TB drives?

Would each 2TB drive also be a new drive letter?

I’m also concerned with overloading one drive letter on v2, I’m using XP Pro.
How much free space should each 2TB drive letter have free?

If you’re using 32-bit XP Pro, then it’s likely you formatted the drives as 2TB NTFS

Since you’d be replacing 2x 1TB drives with 2x 2TB drives in the v2, you’d be going from 2.7 TB to 3.6 TB available physical storage on your Drobo v2.

Since you already have 4 TB of storage provisioned (2x 2TB volumes), you shouldn’t end up with another a drive letter. You’ll just have 900 GB more free, added to your 506 GB, would give you 1.406 TB free between the two drive letters, H and I.

You’ll only end up with a new drive letter when your physical storage exceeds the provisioned storage. That would happen if/when you replace another 1 TB drive with a 1.5 TB or 2 TB drive, because you’d have more than 4 TB of physical storage, so another volume would be created.

Free space is shared among all volumes on the Drobo, so there’s no real amount you should keep “free” on each volume. Just make sure your total data doesn’t exceed your total capacity. Drobo will warn you before that happens.