what's usually lifespan of a HDD

This is dependent on usage. Drives lifespans are measured in amount of data read and written. For HDD's it's averaged at around 4 years.

for desktop computer drives can last for a long time. . i have old hard drives from 2007 i am still using in a desktop computer for storage and have no issues with them.

Based on our personal experience with more than a thousand HDDs of different manufacturers, different types, different capacities, etc., etc...

Forget about "average lifespans" and the like. Understand that this is a crapshoot - nothing more, nothing less.

The lifespan is one day to 20+ years. There's simply no telling. So don't bother about "averages" and the like.

You have one - and only one - concern. To back up your system - your data - on a reasonably frequent/routine basis. Everything else is conversation, capiche?

With HDD's they use a "Phase" system usually for determining the life of the drive. Each "Phase" is 1.5 years and each phase increase's the chance for the drive to fail. But thats generalized, like you said it's a crap shoot.

Like people have said, when looking at individual devices, it's hard to say. Even if you can expect 99% of the drives to last 5 years, there's no guarantee that the drive you bought wouldn't be more marginal or wouldn't fail early or last longer. It's all viewed from a larger population. One thing you can probably expect for really anything is that putting stressors on a product it wasn't intended to handle will likely reduce the reliability but even then it's hard to say. Consumer storage devices are generally spec'd to last about 3 years. I wouldn't be surprised for an HDD to last longer, but I wouldn't count on it.

Now if you want some more details on what can affect a drive, you're somewhat correct in thinking that the mechanical nature of HDDs plays a factor.

Workload is one of the biggest factors obviously. This doesn't necessarily mean amount of data transferred, but rather more on how the drive is accessed. For some examples, lots of random operations and seeks will mean more movement of the mechanical arm whereas if the workload is mostly sequential, then you can crank through a bunch of data with minimal movement. Depending on the design of the drive, lots of spindown standbys can also put more wear on the motor.

Environmental can play a big part as well. High humidity, high altitude, and excessive temperature can all be different stress factors on an HDD. High external shock like dropping a drive is of course well known to be problematic, but vibration can also be a factor depending on the severity and maybe even frequency.

Now for most normal use cases though, I would still expect an HDD to outlast a similar class SSD. While SSDs are not mechanical, it doesn't make them any less susceptible to wear.

On the workload side, due to simple electrical physics, there is a fairly hard defined limit on how much data can be written to each NAND block of an SSD. This is why SSD life is often reported in TBW (terabytes written).

On the environmental side, while humidity, altitude, shock, and vibration are not as big of a factor, SSDs are much much more susceptible to heat. This is again, simply due to the characteristics of a semiconductor type device. What makes the issue potentially worse is that SSDs generate more heat during operation than a HDD, especially for faster NVMe devices. So they can pretty much cook themselves without sufficient cooling.

Monitoring certain attributes in the SMART data is the best universally available signs. Things like reallocated blocks will help indicate a drive that is potentially going to fail due to wear. Additional information may also be available using the specific manufacture's own drive monitoring tool.

Of course there can still be ways for it to fail catastrophically without previous signs, like a capacitor burning out on the board for example.

if your computer gives you a SMART error warning on boot up or somewhere else then it means its about to fail. if you hear clicking coming from the drive then it is starting to or has failed

Also check drive size, as sectors go bad the drive writes a skip in front of the bad sector which will lower your drive capacity. Also running crystal disk benchmark if you feel your drive is becoming slow, drives tend to slow down on their read and write speed before they go bad. If you are worried about a drive you have you might consider a backup or a raid 1