If I want to buy an external hdd for backup-only purpose, which one is a better solution? Pros and cons? For example, WD My Book vs. WD My Passport Ultra. Which one is more durable and reliable?

I'm afraid there is no straight answer to this. There are hard drives build with reliability in mind, but those are used for server purposes usually and cost much, much more. Even those aren't 100% fault proof, and you can get a faulty hard drive no matter what you do. It's just down to chance. Whatever you get, I suggest you do a burn first, before using it. That means filling the drive with zeroes at least once, so that any fault/bad sector that is immediately apparent will make itself known. Most of the problems with faulty drives usually appear on the first hours of using them anyway. On Linux, you can do this with the "dd" command (be careful with it, though).

From my personal experience, I use a portable (2,5" USB 3.0) http://www.techradar.com/reviews/pc-mac/pc-components/storage/disk-drives-hdd-ssd/samsung-m3-portable-4tb-1301619/review Samsung M3 4TB. It's the largest 2,5 portable drive available, and it's good for backup. It slows down if you write a lot of small files on it, instead of large ones, because of the technology it uses (SMR). It is a backup drive though, and that shouldn't matter a lot to you if you use it for that reason.

If you have infinite amounts of cash and some technical expertise, I suggest the He10 drive by HGST. It's a 10TB monster with outlandish performance, but it will set you back 1000 dollars or so. It is of course, an internal drive, designed with servers in mind. But it can be tweaked for home backups. NOTE: A portable drive is prone to damage if you move it while it's in use. Always keep in mind that while the drive is powered on, you shouldn't move it a lot, and for heaven's sake, DO NOT DROP IT.

http://arstechnica.com/information-technology/2015/12/hgst-releases-helium-filled-10tb-hard-drive-seagate-twiddles-shingled-fingers/

Another solution would be to get a RAID box, and set it up for redudancy. You'll need at least 3 identical drives for this, depending on the RAID setup you choose, but it will grant more peace of mind. That too, can of course ultimately lead to hardware failure. You can never, ever account for everything.

In the world of computers data doesn't exist at all unless it exists in at least three different places. :)

I wouldn't touch a WD portable drive with an eleven and a half foot pole. They use some nonstandard workings that they don't bother to disclose in terms of things like power management. Which is probably acceptable if you're using a supported OS, but it's not something that should be rewarded or encouraged.

I've been using a 1 TB seagate drive for years now and it's never given me any trouble. `It's just an expansion portable drive.

I have a bunch of Seagate drives.

The desktop drives come with larger capacities (I buy them at the local post office, 4 TB is the largest they have), but need a power brick.

The portable drives come in smaller capacities, the latest one I have is for backing up Steam and GOG games with 2 TB, but they can be used without a power brick.

So IMO portability is the factor. If you will move the drive around, get the portable one. Having to move the power brick is a pain.

And hopefully your computer has USB 3.0, it makes a massive difference!

3 Seagate portables for me, and they travel (take a little bit of abuse). The oldest is probably 6 years along at this point and I use it at least once per week. The newest one is USB 3.0 and, as mentioned above, that makes a big difference compared with the other two. For day-to-day use where it's only backing up a few files at a time, however, USB 2 should be adequate. For copying your games library, definitely go with USB 3. Note that a USB 3 drive should be compatible with USB 2, in the event that you're on an old PC without USB 3 ports.

If you're using it as an automatic back-up device, then backup software comes into play so be sure to have a look at that as part of your decision. I have my old Seagate set - through the included software - to back-up specific folders automatically every time it's plugged in. Except for connecting the drive it's completely automated. There are probably some free third-party solutions to do the same thing but I haven't needed to look into that since the included manager does the job.

Anyway, I can't complain about the reliability. The only problem I've had has been operator error: I wasn't plugging in the USB 3 cable all of the way into the port on the newest drive. Whoops.

But.. are you going to take the drive around?
If no, the WD My Book is perfect, otherwise you could buy the portable.
However, I've only tried the first type (and even used it on holiday.. on a vibrating ferry-boat lol).

I see. One of those sex-therapy cruises, huh?

He said ferry, not fairy. ;)
I have a WD My Book 4TB and it works great. I also use a Seagate 1TB and it also works well. I bought the latter several years ago and the former far more recently when I needed to transfer my 2TB internal drive upon failure. Neither has given me many issues except if I expect them to be available for extended periods of consistent use (downloading large files directly to it, or uploading directly from it). Both drives work very well.

Oh, I'm well aware...

Probably just get the one that has the features you want.
Personally for backups I go for Size and Connection, 3-4TB and USB3.0.

Buy an Icy box enclosure or hot swappable docks and internal hdd-s, switch hdd-s per need, win.

http://raidsonic.de/products/external_cases/hdd_ssd_docking/index_en.php
http://raidsonic.de/products/external_cases/external_cases_35/index_en.php

I 2nd this, build a NAS with plenty of slots.

I'll third a NAS box, but warn against the 3tb Seagate drives.

I used 4 in a server and after 2 years I had two died in quick succession killing the raid. the other 2 died soon after.

I now used WD drives. 4 in a 8TB server and another 4 in a back-up NAS.

I saw the review on newegg. Pretty low rated stuff.

Thanks for the tip. i'll stick with WD.