Drobo V2, extreme delays but not full

I have a Drobo V2 which presently has 3 x 1.5TB drives fitted, showing three green status lights.

This is set for 2TB volumes, so the first should be the full 2TB and the second showing as 2TB with rather less actual storage space.

It’s got down to about 300GB free on the first volume, with only about 10GB used on the second volume.

Recently, I’m getting long delays every time I try to write a file to the FIRST volume - I guess this is because it’s passed the 85% full mark?

I can understand the idea of delays to warn when a volume is reaching it’s capacity limit on ‘thin provisioned’ volumes, but why when the full space is genuinely available on physical storage?

These delays are not counting the spin-up time from idle.

I’m using this for archiving large files, very little has ever been deleted so I cannot see any logical reason for the delays. Once it has started writing the speed is reasonable, it just seems to take two attempts to write.
I get a timeout / delayed write fail error, then after clearing all the Windows error popups a second attempt works…

Reading files is fine, no delays or problems (once it’s spun up).

What OS are you on?
What firmware are you on?
In dashboard > advanced controls > data tab:
What is showing for used space? The percentage.