Some time ago I made virtual drives from a flash drive using this tutorial. The images (all 3 of them of a same drive) worked "fine", that is to say they did mount, and I was busy solving the problems in this question.
Then suddenly VirtualBox stopped detecting them! Basically I have a triangle with exclamation mark in it next to the virtual drive and the error I get on mouseover is VERR_FILE_NOT_FOUND.
The interesting thing is that all images are copies of one and that one mounted fine. One of the copies mounted fine too and I didn't touch the third at all. Furthermore I didn't change any VirtualBox settings and I didn't edit the files themselves in any way.
So how do I find out what's going on?