Just thouht I'd drop by and see if there was any answers to this. About a week and not a single thing. This type of response was why I swapped products. Unfortunately doesn't look like much has changed. Sad to see Fusion in such a decline. At least their are alternatives.
This is a user-to-user forum. While VMWare participates here, there's no guarantee of a response.
For what it's worth, I (and hundreds of my colleagues) run VM's off external SSD's all the time without issue. It's likely something unusual with your setup rather than a defect in Fusion.