Thought it was a fluke but I can’t export with the Arranger active. Is this a known issue…?
I can flatten it but it’s a PITA.
Is the projectaudio on the OSX systemdisk?
If yes… move the project to other HD.
A little more info in your signature could help 
Project is on an internal SATA2 RAID0 striped array, separate drive for OS, and exporting is to an external FW800 RAID0 array. Trust me when I say it’s not an actual overload
I realised what is happening though - the arranger will stop playback when it reaches the end of the arrangement, however if the right locator is set to a point at or past this, then the arranger will stop the export short, but Cubase will wrongly report it as a disk overload.
Technically this is a bug - a minor one, but still a bug. Cubase should either not permit export under these conditions, or should simply gracefully stop the export at the end of the arranger chain, ignoring the right locator.