In theory, if you submit a computer for repair with a major module such as the hard drive replaced by a third party component, then you're out of warranty coverage and the repair facility doesn't have to cover the repair. Enforcement of this may be spotty, but it's worth being aware of.

Again, if your data is private and you don't want someone seeing it, then make a backup and erase the drive before the repair. If it's really private and you're afraid of them using tools to try to recover erased data -- hint, they won't, but if you're paranoid you're not going to believe me anyway -- then use a multi-pass erase to scramble & zero out the disk. And if you're really really paranoid, then go ahead and swap the drive, but just be ready for the possibility that the repair will be declined.

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DO NOT LEAVE IT IS NOT REAL