Kirk Hammett has "lost" 250 new Metallica riffs.
The 'Enter Sandman' rockers have been working on a new album but progress was hindered six months ago when "something very unfortunate" happened to the guitarist and he mislayed his phone, which contained all his musical ideas, that hadn't been backed up.
He said "When it happened, I was bummed out for about two or three days. I just plain lost it...
"To try to remember those riffs? I can only remember, like, eight of 'em. So I just chalked it down to maybe it just wasn't meant to be and I'll just move forward with it."
While Kirk insists he's moving on from the lost ideas, he hasn't given up hope the phone will reappear some day.
Speaking on Hatebreed frontman Jamey Jasta's podcast, he added: "I'm still looking for it to this day. I just set it somewhere and ... it still might turn up. I'm hoping it will.
"All you musicians out there who use your phone, make sure it's backed up. Right?!"
Though 250 riffs may be missing, Kirk promised the record - the follow-up to 2008's 'Death Magnetic' - is still "super riffy" and "super heavy".
He added: "It's a lot similar to 'Death Magnetic', but different in certain parts. James [Hetfield] is doing a lot of really, really cool melody stuff these days, a lot of vocal layers. 'Lords of Summer' is a good example of that, the beginning."