I had to temporarily disable Avast from offering to scan attached USB drives but now I can’t figure out how to re-enable it. How do I do so?
I not entirely sure where you did this nor where Avast ‘offered’ to scan attached USB Drives.
Sound like something that might be offered in the Smart scan if you happen to have a USB Drive attached.
But you might be also able to add it to the UI > Settings > Protection Virus Scans > Targeted Scan and add the drive letter.
Though I’m not entirely sure of the benefit of this (as it sounds like on-demand scanning), rather than the File System Shield an on-access scanner that should scan activity wherever that may be.
Settings → Notifications → “Ask to scan external storage”
Apart from this one, there is also another auto-starting USB drive scanning option, which is found in the security settings under “file protection”.
Look at the attachment.
The above is only for anything that is set to automatically run once you plug it in, like an auto-run .exe. That one is on by default, but enabling that doesn’t cause Avast to ask any time an external drive is connected, regardless of what’s on it.
Yes I know this. 
Just wanted to give a little more guidance on USB drive scan settings.