Well, interesting findings over the past few days... it's not whether the file exists, it's all down to HyperBac compression...

HyperBac off: Backups 400GB, 1.25 hours, read speed 100MBps, write speed 100MBps. (roughly)

Hyperbac on: Backups 80GB, 2.5 hours, read speed 50MBps, write speed 8MBps (again, roughly).

This is not the performance I've come to expect from HyperBac, anyone seen anything similar?

(As it's now a product-specific question, I'll also post on the RedGate forums, but very interested to hear any similar experiences here)