did it for 2 reasons.
1. ordered placement is based on age of drives. i'd prefer the oldest in the pool to have the easiest life, so be read from only. whilst newest gets written to until newer one arrives on the scene.,
2. since storage in the last 2 months has gone under 1TB free, when i started to get the "disk full" errors from windows due to the pool space spread too thinly, until the new drive is added i decided to then utilise the plugin.
still don't know why it gives me the error, plus file transfer speeds have slowed down as well. so will need to get a new drive in ASAP.