choice of external USB drive for mount at boot

As I've mentioned previously, I'm working on a home server Pi (2 B)
project, & I'll need to attach a USB hard drive & configure it to get
mounted at boot (to provide /var & /home/adam at least --- maybe
everything but /boot, although I haven't decided on that yet).
Anyway, it seems to me (but I'm open to correction) that externally
powered (i.e., mains plug) USB hard drives have disappeared these
days. I get the impression that running a big USB HD off a Pi's USB
port is a bad idea, & I'd like to go for reliability here.
Should I keep looking for a USB drive with a mains plug/adapter, or
just use a powered USB hub between the drive & the Pi?
Thanks.
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Reply to
Adam Funk
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While 2.5" drives seem to be USB powered, there are still many wall-wart powered 3.5" drives.
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-michael - NadaNet 3.1 and AppleCrate II: http://home.comcast.net/~mjmahon
Reply to
Michael J. Mahon
I would highly recommend a drive enclosure with external power. If you can't find one for a 2.5" drive, many 3.5" enclosures will also take a 2.5" drive. Or there are adapters that don't have an enclosure and you can make your own case.
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Rick
Reply to
rickman
By "enclosure", you mean buy an enclosure that plugs into a USB port & a separate hard drive (e.g, SATA) that fits in the enclosure --- as opposed to just buying a portable USB drive (with external power)?
Reply to
Adam Funk
OK, I'll keep looking. I guess the powered hub is not as good?
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Reply to
Adam Funk
Yes. When I read your post I just had in my mind an enclosure that you put a hard drive into. I see now you are talking about buying the entire unit with the hard drive. I expect they will be USB powered for the most part, but that usually means a Y cable with two plugs to go into your computer. I looked them up recently and every hard drive I found needed about 1 Amp while the USB port spec is for up to 500 mA. I'm a bit surprised they will even work on a Y cable, but I guess the USB ports don't hard limit at 500 mA.
Regardless, I am pretty sure a rotating hard drive will not work well powered off the rPi. Mine resets when I plug a USB serial port adapter into it. I'm pretty sure its not the power supply. The serial port device only draws around 50 mA. lol
Another thing you can do is to buy a powered USB hub. I got a 7 port one pretty cheap on eBay and the power supply for it is so cheap it won't maintain voltage with just a keyboard, mouse and serial port adapter. So I'm buying a 5 Amp supply from eBay that looks like a small laptop power brick. Then I can plug in a rotating hard drive or even two.
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Rick
Reply to
rickman
Drives described as "desktop" are 3.5" drives and come with a wall wart for external power. Drives described as "portable" are 2.5" and USB powered. Here is part of one manufacturers range of desktop and portable drives . Note the "package contents" of the desktop drive includes an AC adapter.
Reply to
Gordon Levi
I expect the Pi reset is caused by the momentary current surge to charge a filter/decoupling electrolytic capacitor in the serial adapter.
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-michael - NadaNet 3.1 and AppleCrate II: http://home.comcast.net/~mjmahon
Reply to
Michael J. Mahon
in Tesco (Hexham) today.
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Cheers 
Dave.
Reply to
Dave Liquorice
Which model of Pi is that? The A+/B+/2B all have better voltage regulator circuits and a higher rated fuse. They also have USB current limiting, set to 600mA by default, with an optional 1200mA setting. That's total USB current, not per-port current.
The A and B models did have problems with hot-plugging many USB devices due to the inrush current, but the newer models are much more tolerant.
Reply to
Dom
The easiest way to be tolerant is to put a 100 uF capacitor on the Pi...then a 10 uF on a plug-in only causes a 10% voltage spike. ;-)
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-michael - NadaNet 3.1 and AppleCrate II: http://home.comcast.net/~mjmahon
Reply to
Michael J. Mahon
This is a model B. Actually, the voltage drop doesn't work exactly that way. I'm sure there is no shortage of capacitance on the Pi and in the PSU. The trouble is the inductance and resistance of the power traces. There are specs for exactly this aspect of the USB interface, so unless the USB slave is designed very badly it shouldn't crap out the rPi.
This is not such a big problem. I have a 7 port powered hub just so I can plug in devices and not whack the rPi. But I've been having stability problems with this thing that I can't get a handle on, both with the dongle powered by the USB serial port and the rPi itself. I am using an HC-05 bluetooth module and it acts very oddly. Either it won't respond to commands over the serial port at all, or when it received a command it sends its response over and over until another character is sent. The rPi seems to intermittently drop keypresses, even when I use the Alt-Tab to switch between tasks, it can get stuck with the task switch window up but not fully switch to the other task when I release the Alt key. I've got a larger than needed supply on the rPi. Maybe I should put the oscilloscope on the power rail and make sure there isn't ripple or whatever.
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Rick
Reply to
rickman
Oh, I understand from reading up on it that that's a bad idea. I'm trying to get a handle on which alternative to use:
1. powered USB hub with "portable" USB drive,
2. externally powered "desktop" USB drive, or
3. externally powered enclosure with USB cable & a separately purchased hard drive stuck in it.
Are the "desktop" USB drives better, longer-lasting, or more reliable than the "portable" ones? Is a drive that I put in an enclosure better than a factory-sealed desktop USB drive?
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Reply to
Adam Funk
I guess I haven't seen any "desktop" models for a while (well, I have one that was surplus at work, but it's "only" 150 GB) & didn't realize that was the keyword to look for. Thanks for clearing that up!
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Reply to
Adam Funk
Is there really any difference between 2 and 3? That is, apart from price and ease of opening the enclosure.
I have a 3.5" desktop USB drive + power brick (currently disused) that I used for about 5 years for overnight backups until increasing backup sizes meant it became too small (120 GB). I thought about swapping the drive for a bigger one since the enclosure electronics do exactly what I wanted, e.g. spinning down when not being accessed. However, the thing is glued shut and will probably be badly damaged if/when I try to open it (its a 2005 vintage WD enclosure - any tips for opening it without wrecking it gladly received).
Similarly, I used to use a sealed 2.5" portable drive for weekly backups (portable so it can be stored off line and safe from fires, mains spikes etc) which was fine until the enclosure electronics failed about 4 years ago. I opened it, discovered that the drive was fine, and put it in a Startech enclosure where it too was used for several years more it ran out of capacity (120GB) a year or two back. Currently the Startech enclosure is still in use with a larger capacity disk and I have a second similar setup (separately bought 2.5" drive + USB enclosure) to provide two backup generations.
My conclusion? Buy the enclosure and drive separately. Doing that can be quite a bit cheaper than buying a sealed consumer package, you get to choose the make of both items and either enclosure or drive can be easily replaced: two screws hold the enclosure shut and the drive plugs in[*].
The last enclosure only anchored the drive by its connectors leaving it free to flop about inside the enclosure, but sticking a foam pad on either side of the drive's other end made it a nice push fit into the enclosure. Pritt pads are good for this: leave the tab over the stickum on the side facing away from the drive so the pads slide into the case without sticking to it.
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martin@   | Martin Gregorie 
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Reply to
Martin Gregorie
I'm going to come to the opposite conclusion that Martin did, with all due respect. I have mostly bought separate enclosures because I wanted to connect a drive from an old computer. I also bought a 3 TB drive in an enclosure to use as a backup. As Martin indicated, the 3 TB drive has features I like including the backup software. The separate enclosures have none and were all made to sell at the lowest price. My 2.5 inch enclosure at least came with a Y cable so it can be powered from two USB ports at 500 mA each. Another 2.5 inch enclosure I bought has just a straight cable and so will only work on a port designed for higher current. It uses a USB 3.0 interface which allows up to 900 mA (a bit less than many 2.5 inch drives) and other updates to the standard allow even higher currents. But if they have added the extra $0.25 connector to the cable it would work with many more computers.
I guess my point is that the separate enclosures are mostly pretty low end products. I think you will get what you pay for with a name brand combined device. You can go to the manufacturer's web site and get detailed info on the product. For a backup/server device I think this is worth a couple of extra bucks.
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Rick
Reply to
rickman
Fair comment, though as it happens, both the cheapie 2.5" enclosures came with dual connectors on the host end of the USB cable. I quite like the quality of the Startech stuff and even the really cheap enclosure is of acceptable quality. In addition, its USB socked is a full-sized one rather than a mini socket - something I rather like.
I don't care about the commercial backup software because it probably won't do what I want:
- the drive I didn't mention does an overnight backup every night. These are compressed tarballs and the current 750 GB drive holds 15 of them, with the oldest backup(s) being deleted at the start of the run until there's enough space to take the new one. Usually its a matter of 'delete one, create one' but backups do get bigger, so every so often it will delete two.
- the other backups are made with rsync and each disk holds data from my RPi as well as from another couple of Linux boxes.
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martin@   | Martin Gregorie 
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Reply to
Martin Gregorie
1. Will be using a 2.5" drive designed for use in a laptop. IMHO these drives aren't really up to heavy use.
2. Will use a 3.5" drive, this is likely to be a more robust drive.
Well you get to choose it rather than not knowing what is in there until after purchase. Then seeing if it reports what the physical drive is. If not you'll have to get the covers off...
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Cheers 
Dave.
Reply to
Dave Liquorice
I don't know what you base the idea on that the 3.5 inch drives are more "robust" than 2.5 inch drives. I expect they are largely the same, but 2.5 inch drives are intended for portable apps where they take a lot of abuse. They are also used in rather low end laptops where they get heavy use due to the paging of limited RAM. I'm not trying to say 2.5 inch drives are more robust, but I am saying that I have never seen any indication that consumer oriented 3.5 inch drives are better.
Hard drives are always a bit of a crap shoot mainly because they are the least reliable part of any computer. Get one that is a little worse than the norm and it can be a problem. Get one that is better than typical and it can still be a problem... as I said, it's a crap shoot.
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Rick
Reply to
rickman
+1
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The Natural Philosopher

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