First, I don't randomly boot from my own usb drives before checking them in a booted OS first. Hell there could be drivers or a BIOS updater on there that I forgot about. It's stupid to *assume* a USB drive is EVER safe from a security stance. With flash drives you either A) know everything that's on them at all times and never loan them out to others or B) assume no flash drive is safe and check it prior to having it in at boot. I prefer the second option.
Secondly, I'm the one that handles all of her tech stuff like laptop at home. All files are stored on a centralized WHS machine that is backed up (and has a copy stored on a HD in the storage unit that I backup critical files to every month or so). So because of this she doesn't have anything on her laptop that would be lost from it. Oh, and her laptop isn't set to boot from usb devices in the BIOS for this reason as well. I change all PC's of mine to boot from HD first and disable all other options once it's up and running. It takes an extra 45 seconds to change it if I need to reformat in the future, and saves me hassles from booting from USB/CD/DVD drives when I forget something bootable is connected to the system.
Finally, it's not intentionally "destructive" where malice is involved. If I had known she was going to leave the flash drive in at boot, then I would have warned her to make sure to not have it in at boot because it might have other stuff on it (since I know that I have used those drives for that in the past). I made the mistake of assuming she was using it on already booted systems to transfer files between them, which wouldn't have mattered if I had a bootable part of the drive or not.
Very true I'm sure, but not for the reasons listed in this thread.
Perhaps you skipped reading the thread, but I acknowledged that it was my mistake for letting her borrow it without checking it knowing that I have had unattended slipstreams on flash drives in the past. However, I had to leave for work (she asked last minute) and I didn't honestly think, "hey what if she has the flash drive in the system at boot on her work system where the IT dept allows bootable usb drives?"