At this scale the right answer would probably be to use a NAS or at least a DAS rather than connect disks directly.

@Giacomo1968 - Thanks for reply. My motherboard is relatively old. There are only USB 3.0 ports. But after learning all these technologies, and considered the length requirement, and thanks for your webpage link, I think converting to USB 3.0 will be likely my solution. It adds hot swapping feature too, which is sweet. More details are updated above.

@gronostaj - Thanks for reply. I didn't heard of the term DAS. But in fact I am already doing a DAS, via SATA cables. I think I will change to USB cables. More details are updated above.

@SamForbis - Thanks for reply. In fact I am building this PC to act as a NAS for my other devices. I do not like those pre-built NAS packages, as the number of bays are so limited. Extra bays are ridiculously overpriced. It is indeed just a plastic box, with some cables and a fan. In addition, they will be less secure than my Linux file server. LBNL they are less possible for me to do customisation & programming. I plan to build on ZFS with LUKS. I had a Orico 2-bay RAID1 enclosure too. But it also had the disconnect problem. It seems not so stable to me. More details are updated above.

If you're building a NAS, consider a multi-bay NAS-centric case or used server-grade disk shelves. You need reliability and loosely hanging cables are not reliable.

@gronostaj - Good point. Thanks again!! Especially I am planning to keep a cat also. I am still figuring how to solve the hanging cables problems - LOL

@gronostaj - But using a NAS case seems not so feasible also. I have to put the NAS case under the desk, which is not so convenient to swap HDDs. More importantly the motherboard seems cannot be mounted inside the case. I have searched for other similar NAS chassis also. Most of them seems quite "raw" and again overpriced (especially with shipping cost added). Yet your point (no dangling cables) is still very good. I think I will consider using the USB approach mentioned in my edit. And get a better "cage" holding my HDDs on desk. There will be only a few USB cables bridging from PC to HDDs.

USB is not a great idea, see What would happen when USB devices draw more power than the hub can provide?.

Also: you're not going to be swapping these disks daily. Unless you'll be running >8 disks, this 1 or 2 occasions a year when a disk fails and you have to dive under the desk shouldn't be that much of a hassle. If you need something that will fit a full-sized motherboard, Fractal Define 7 seems to be quite popular since it can fit 14 (!) 3.5" disks.

@gronostaj - Thanks again!! In my case, I have dozens of small size HDDs and I do need swapping around, once a few days, or even a few times in a day. Thanks for referring to another question that USB hub is not a good idea. I did forgot about the bottleneck of an USB hub. I should not use that 7-port hub when I need better transfer speed. But the hub I quoted is 48W I guess power is not a problem for it.

@gronostaj - Sorry I still cannot see what is the huge benefit to buy a pre-built DAS (other than a better chassis). In most DAS there are only 1 USB cable connecting to the PC. But in my setup I can have every dedicated USB cable from each HDDs to each PC USB port. Of course I need to have some cable management coils in between. The Fractal Define cases are really nice and very flexible!! Even small size "Node" cases can house 6-8 HDDs. I will consider them also. Btw I am using Corsair 280X with mATX motherboard.

Usually the goal of building a multi-bay NAS or a DAS is to have it manage the disks as a single large volume with redundancy. So eg. 6 out of your 8 disks offer usable space and 2 disks store parity data. If any disks dies, you can swap it for a new one while NAS is continuously available and operating uninterrupted. Such redundant array can survive loss of up to the number of parity drives (including failure of non-parity drives).

If that's your goal, then you need reliable disk connections. Once any disk drops from the array, you can't simply reconnect it and carry on because the array could have changed in the meantime. This means that each disconnected drive is treated as a completely new drive and rebuilt (= merged into array to replace the previous one). It's a long process that stresses other disks in the array too.

The problem with USB, eSATA, loosely dangling SATA etc. is that its connections are unreliable and you may be triggering unnecessary rebuilds. Also in these conditions you can exceed the redundancy limit and lose the array.

Hmm... i do get your point. I think in my case, i am building several sets of RAID 1 arrays. If i have enough ports and spaces, i can actually keep them all 24x7 connected. But some of them doesnt need 24x7 available so i will switch some off to save the HDD lifespan. In reality i wont have enough ports and spaces. So i need to swap different sets of RAID 1 arrays.

In another superuser thread suggests that USB is not designed for permanent connection. However DAS data is connected via USB also.

With RAID1 it's the same deal: once a disk drops, it will have to be rebuilt once its back and this process can take hours or days, depending on disk size and performance. If it drops again during the rebuild, it will start over. If the other disk drops, your array crashes, you have to recreate it and restore data from backups. USB is just not suitable for RAID.

Also, you're not saving disk's lifespan by disconnecting it, quite the opposite. HDDs don't mind spinning 24/7, but failure rates are significantly higher when disks are spinning up. For maximum reliability you should keep them connected all the time. Spin-down is only useful for power saving.

USB is good enough only if you want to use these disks as individual drives or have them virtually pooled with something like mergerfs. For anything that operates in an array you need a reliable, permanent connection. For this number of drives you'll very likely run out of SATA ports and you'll need a HBA card or a RAID controller configured to work in passthrough mode.

Can you share what's your goal with this project? I've been dabbling with this topic for the last few months, maybe I can suggest something.

Currently I am using Windows and mirroring manual, which is very tedious and prone to error. I really plan build a NAS with Linux on ZFS. For example I will create five zpools (A, B, C, D, E) and each zpool is RAID 1 inside. A houses the OS, which is likely online for 24x7. B consists of two 8TB relatively new HDDs containing my photos and videos. C consists of some less important data, for example bit torrent movies. So I will use four old 1TB HDDs as RAID1+0 to get 2TB spaces.

D may contains some archive data, which D is the one that I do not turn in on for most of the days.

In Linux, I suppose I will be able to set zpool D offline, before unplugging all the USB HDDs belongs to D.

So it wont trigger a disk drop and rebuild. Unless the USB connection will accidentally disconnect by itself. I suppose a reliable USB cable and USB converter will not allow accidental disconnection to happen.

I also understand HDDs lifespan depends on the start/stop count. But mechanical HDDs do have wear. online time is also one of the factor which affects its lifespan, may be just less deterministic. In the above example, zpool D are the archive data. Keeping it offline will be a better setup.

@gronostaj do you have reliable date on the up/down spinning issue? My past sources have split 50/50 between one or the other.

@Albin yeah i do understand that other than RAID I still have to do some offline backups. RAID only avoid data loss when HDD failure occurs. Backups avoid human errors and in case of system failure we can rollback.

@Albin by the way those are just examples to illustrate that I do need a few different zpools, and not all the zpools are online 24x7.

@Albin I've only seen anecdotal evidence so far I think

@midnite One more question you've asked but I didn't answer it earlier: the difference between a single-USB DAS and your multiple USB-connected drives is that DAS is autonomous to some extent. Accidental disconnection may interrupt transfers - something any competent filesystem should handle gracefully - but won't affect array integrity.

@midnite actually RAIDs are not suppose to avoid data loss at all (if they do it's rather a "last resort" if a continuous backup is not feasible so you have at least an "incomplete" data loss protection). But I'm getting out of the "consumer realm" here...