Purple band suddenly appearing on layered TIFF files

Ah well, you live and you learn. Appreciate the insight and help, all!

Please post a system report. Help->System Info

The big problem with external drives is connection stability. External drives get kicked around, and cables and connectors inevitably get worn and increasingly loose-fitting. It only takes a nanosecond broken connection to corrupt a file.

In addition you have an extra layer of USB drivers and protocols, making the save process much slower and vulnerable to random errors.

https://helpx.adobe.com/photoshop/kb/networks-removable-media-photoshop.html?x-product=Helpx%2F1.0.0&x-product-location=Search%3AForums%3Alink%2F3.4.0 

Generally, when using external drives, handle with care. Don't touch it while data transfer is in progress. Replace cables regularly.

@defaultyfk3qa3z1x64 too late in hindsight but its recommended to never save directly to an external source - drive, server, cloud service etc. Always save local and drag over to store. Sorry.

Agh, bummer. I'm saving directly to an external drive. Perhaps I should work on the file locally?

Not the worst news, as I can copy over my layers to the original scan (unless there's any corruption connection there).

That looks like file corruption, which is always caused by failing hardware.

Are you saving directly to an external drive, or over a network connection?