A friend of mine plugged in the wrong adapter and fried her external drive. I opened the case, took out the HDD and checked the PCB. I know of the D3 and D4 5V712V diodes but here there isnt anything printed on the PCB!

There are 2 components that look like TVS diodes near the power connector on the PCB. One is marked something like DUI C824 and the other one is burnt but I can make out KVD at least.

Anyone familiar with these PCBs?

If you attach a close-up photo(s) of the burned PCB (especially the area of the damaged components), that would probably help people to answer your questions correctly - if you're accepting the risks of trying DIY...

Depending on the amount of PCB damage, it might be more successful to investiate the possibility of replacing the PCB (and everything which that process requires). Damage elsewhere than the PCB is a possibility too.

Here are a couple of pics. The burnt component is to the bottom left. If this is like the D3/D4 on other PCBs, can I replace it with a VISHAY SMBJ12A/SMBJ5.0A?

You can start by removing or replacing the diodes and check after.

My main question was if this actually was the 5V/12V diodes or not, so based on your reply I guess they are.

Thank you once again for the help.

It is indeed a TVS diode and removing it should solve the problem, unless other components are also damaged. If you try and power it up after removing it make sure that your power supply is 100% as you have removed the protection from the PCB. Just be careful.

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Not only.

If something else is damaged and not working properly this can cause more damage when you turn on the drive.

Good luck.

Ok, well can't I replace it with a VISHAY SMBJ12A or SMBJ5.0A? Can you see which one is burnt (5V or 12V)?

From the picture the left one is KO for sure.
The concern is the rest of the circuit (to be tested) but don't want to say nothing about it . Just be aware that if you fry preamp or ROM what it was let's say "100" becomes "500".

Yea, I mean if you can see if the left one is 5V or 12V. I want to know because if I instead of only remove it also replace it, then it would be safer to run, wouldnt it?

I have both VISHAY SMBJ12A and SMBJ5.0A diodes from Farnell at home that I can use.

To answer your question, it's the 12V TVS (you can use the SATA power pinout, if you want to confirm this answer).

IMHO it would make sense to use a multimeter to check the 5V TVS while you have the PCB removed.

Note the risks mentioned too, and take all possible ESD precautions.