Ever since losing a few photographs about 6 years ago due to hard drive failure I've been a bit of a backup fanatic. Up to now, everything seems to have been working well.
On my machine, my Lightroom import folder is continuously backed up to a network drive and an online service along with my network archive of photographs.
After import I usually go through and reject photographs I don't want and periodically delete these photo from my drive. However, the next time I import from memory card, Lightroom highlights these previously deleted photos as importable again, so I usually uncheck all and import only what I want. I managed to train myself to assume some photographs have already been imported and rejected, when in fact, they may never have been imported!
I lost about 14 photographs, because I never imported them from my SD card. After a few photoshoots, these unimported photos got list in the noise of 'deleted photos' and then I formatted the SD card - Oh no!
A simple workflow change will sort this, either:
a) never delete photographs; I don't see the point of this, if I reject a photograph because I have another, better version, then deleting is an essential housecleaning task.
b) move deleted photographs to another folder, this way Lightroom retains knowledge about them, but I can exclude them easily from my library.
c) always synchronise the cleanup task of deleting rejected photographs, with a format or 'delete all' of my SD card.
I'm going to go for c) for the moment.