Can I stop external HDD's sleeping?

Setup as follows;

2018 iMac with High Sierra

2x WD My Book Pro's (RAID1, connected via Thunderbolt - Thunderbolt 2 adapter)

Specifically focussing on BackBlaze upload

Basically, when the iMac goes into sleep, the external drives stop spinning, which in turns stops the upload to BackBlaze. I've noticed when the iMac goes into sleep, the internal HDD stays awake until their upload is complete - is there any way I can replicate this on the externals? I can't find anything in settings to do this, other than to keep the iMac fully awake throughout the night - something I don't want to do.

As I put usually 150GB of files onto the external drives in one go (photographer) - it'd be ideal if the upload would complete overnight.

Many thanks in advance.