If there is one important thing that could be learnt from reading stories written by employees and owners of computer repair shops, it is that such shops cannot and should not be trusted because a fraction of people working in such places are more than happy to eagerly analyze the entire contents of their client's file system, even if such analysis is not in any way required to solve the issue they are being paid to fix. I am not denying the fact that there are a lot of honourable and professional people working in such places who respect computer's owner right to privacy, but we still need to remember that the number of people who could be described as the opposite is greater than zero. Even if such people make as little as 1% of all the employees, it is still a great concern. Such people will feel absolutely no shame whatsoever while rummaging through your private files, for reason no other than that they technically (but not morally) can, and will potentially publicly upload, let's say, your photos to the Web anything that could in their eyes make a "funny" juvenile Internet "meme" of the "super embarrassing private thing found in client's PC!" flavour.
Only sensible solution is, never leave your HDDs/SSDs together with your PC. If your PC has CD/DVD optical drive, set the BIOS to boot from it. Obtain a Live CD, on which a portable version of operating system of your choice is burnt. Before leaving the PC in repair shop, remove all HDDs/SSDs and instead put the said Live CD inside the drive. Live USB stick could be used instead, but it won't have the advantage of being read-only as CD/DVD.
Good thing to do, regardless if you want to do according to my aforementioned advice or not, is to make a subtle and implicit but unambiguous impression that you were a lawyer or otherwise professionally involved with occupations that suggest having a significant legal muscle. Practicing law without the license is illegal, but dressing like a lawyer and using lawyer slang is not, as long as you do not officially and explicitly claim that your are a lawyer while not being a lawyer. Any subtle suggestions of having family or friends employed in state's administrative occupation, like an embassy, would also certainly discourage any bottom-feeding wannabe digital sniffers from messing with you from their fear of being sued. The mentioned impression and suggestions could be both in the form of your clothing and appearance while you are visiting the shop and interacting with employees, and in the form of suggestive files with appropriately intimidating contents, like "nondisclosure_breach_lawsuit_2.docx" innocently and seemingly accidentally left in plain sight on your PC's desktop. In general, figuratively speaking, just make sure to subtly flex your legal muscle in front of the repair shop crew, whether that muscle is real or non-existent, to make a proper impression and indirectly telegraph that messing with your privacy will have consequences.