Without any information about what went wrong when Bitcoin Core invalidated it, it'll be hard to pinpoint the problem especially if your hardware is doing fine.

However, I'll consider USB external drives far less reliable than SATA ones. The controllers that I've had for my drives were not exactly the best though they were fairly expensive and well known. Bitcoin Core tends to magnify the problems associated with them due to the nature of the program and how it interacts with the disk. Assuming that your data directory is on the external SSD.