How can I prevent more broken hard disks/other hardware and what should I do to detect the problem?

You can't.

The only reliable solution for data loss is having automated, known-good, restorable backups. You PSU can fail, cables can short, disk may die due to manufacturing issues or old age, malware can delete or encrypt your files and finally, you can simply delete them accidentally. Backups solve all of these problems.

Follow the 3-2-1 strategy: 3 copies using 2 types of media, including 1 off-site copy.

Your questions

1. Can I safely assume that my modular PSU cables are allright and should I check if there are other problems?

Either the cables are shorted or the PSU is. Frankly, I'd probably get a new one and wouldn't trust this one. You can donate it to someone who is willing and knowledgeable enough to check if it's fine and fix it if necessary. It's their risk then.

2. Can broken SATA data cables can cause a broken hard disk? I never heard something like that.

Unlikely. SATA data cable doesn't use high-power signals.

3. Can a hard disk just create a shortcut itself and take another hard disk with it? If that’s the case, I think I can safely rebuild my pc.

It can. PSU's short-circuit protection should kick in and save the other disk (and everything else that's powered by that PSU rail). It's not guaranteed to work, but it's better to have it than not.

Again, I wouldn't trust this PSU. You can make the assumption that it's disk's fault this time, but if this assumption is wrong, you'll fry another disk.

The most important question

4. Can you add to much hard disks to a PSU? I have 5 HDD’s, but an Corsair PSU of 750 Watt, so I don’t think that is the problem?

That's a very important question.

HDDs don't draw that much power under normal operation - generally less 10W per disk, possibly less - but they do draw a lot more when they're spinning up. Based on my experience I'd assume 30W per disk is required.

Now, a proper server would spin up disks one by one to avoid power spikes. You don't have this kind of hardware, so your 5 disks will draw up to 150W at spin-up. The PSU should sustain this if they're connected properly. We'll get back to this in a second.

Assuming all connections are fine, if the PSU wouldn't keep up with power requirements, the voltage would drop and disks wouldn't spin up properly immediately or at all. It's far from an ideal situation, but it shouldn't damage electronics (they could suffer mechanically though).

However, how your disks are connected is crucial. We're focused mostly on the 12V line because that's what 3.5" HDDs use.

• SATA power connector has three 12V pins, each capable of delivering 1.5A, that's 18W at 12V. 3 × 18W = 54W. That's sufficient to power one or maybe two disks.
• Molex 4-pin connector provides 11A, that's 132W. You could theoretically power 4-5 disks off of one of these.

However, to power more than one disk from a single connector you need a splitter. Splitters aren't usually high quality and they're known to cause shorts and even fires. Why? Because low-quality splitters use molded plugs and in the process of molding connectors can move towards each other. There's a slight resistance between them that slowly chars the plastic. Eventually this charred plastic shorts two connectors together and fries your disk.

The same applies to SATA-Molex and Molex-SATA adapters. In addition, Molex-powered devices can expect more power than SATA power connector provides, so Molex-SATA adapters are dangerous by design.

See this video for an autopsy of a fried Molex adapter.

Try to use only cables included with the PSU, avoid splitters and adapters. If you have to, at least don't buy molded ones. Distribute disks between PSU rails to limit fatalities in case of a failure - you want the least number of devices per rail, so when one goes bad, there's a fewest possible number of devices sharing a rail with it. Rails should be described on PSU's label.

Buying a good PSU

Don't buy no-name and cheap brands. PSU is not a part to skimp on. If the PSU goes bad, all of your components can be fried. I've personally lost 2 HDDs and a motherboard to a bad PSU. It's just not worth it to save money there, it will just cost you more money later.

Corsair is a respectable brand, although not all of their product lines are equally reliable. Google around, read reviews, watch competent videos on YouTube. JonnyGuru is the ultimate PSU authority. Learn how to find out if a PSU is totally crappy (this weeds out stuff that you must absolutely avoid, but won't rule out low-quality brand name units).