Audio_Freek,
Is the same life span of 3-4 years applies to SSD EHDD as well?
-John.
The key difference is that a hard drive has a mechanical head (not unlike a turntable) that reads from a spinning disk. Due to the mechanical nature of the operation, mechanical degradation will happen naturally over the course of time (unless of course you don't use it at all). Meaning - it doesn't really matter if you are reading or writing to the disk.
In fact, it is a testament to man's engineering accomplishment that hard drives last so reliably when you consider that the platter spins at 5000-10000 rpm, and the head hovers millimeters above this fast spinning disk.
What *will* make a difference is if you keep your drive defragmented which will ensure that memory blocks are stored contiguously and means that your head doesn't have to go back and forth unnecessarily to read a single song.
Please note that different drives are engineered at different tolerances and with different reliability metrics. Just like audio components, you get what you pay for (largely). So a more expensive drive (especially the "enterprise class" drives) with a higher MTBF rating will generally be more reliable than a drive that is the cheapest you can buy (which is what people generally do).
While drives do fail, it is also very reasonable to have a well engineered drive last 8-10 years depending on usage, or even more.
It is also easy enough nowadays to have another external drive or even a pen drive purely as a backup.
SSDs are completely different though. They degrade over time because of write operations - i.e. when memory blocks get overwritten time and again. They also employ some "wear leveling" techniques to mitigate this to some extent. Since music libraries are more often read, and very seldom written and rewritten constantly, SSDs should theoretically be much better than regular hard drives. If you want the best of the best, go for SLC based SSDs. They will be more expensive of course. Or most SSDs from Intel should give you a better level of reliability than you would get from most other options. Intel just does a much better job of testing and validation.
However, note that good quality SSDs and good quality hard drives have both similar MTBF values - about 1.2 - 1.4 million hours - which is 100+ years. So you should actually be safe with both.
Examples:
http://ark.intel.com/products/56569/
http://www.amazon.com/Western-Digital-WD5002ABYS-3-5-inch-Enterprise/dp/B001EMZPD0
http://www.wdc.com/en/products/products.aspx?id=20