Everything? to know about external HDD.(or  SSD)          

This device is used for 2 reasons, Backup data or to Clone a HDD to HDD or to SSD. 
The time to backup is before your main boot drive gets corrupted !
I use TODO backup (free) to backup all my data. (It works very good, even to recover just one file later, not all at once!) URL link to TODO.

The external box, or enclosure, or a DOCKING STATION ,  or just  a clone cable.    (there  are lots of choices)
Do know that some folks avoid all this and use cloud storage. (I use private servers local and remote to back up my data)

This page below is not about turn key, USB backup pocket drives or Backup Plus(tm) or the like drives already fitted to a case (that mostly just work ok on modern PCs, but not the 100mA version below:
My page is about all PCs working with  the best external drive you can make your self. (buy the case, then buy a drive you like and be happy)

Rule #1 is for Hard disk drive enclosure limits  (not SSD) is power and current.
Your PC
will not have enough power on the USB port to run 12vdc bus.
HDD  startup current range from a low of 1 amp to 2 or more amps on 2.5" and on 3.5" the  amps are way more.
My  ST2000LM007 2TB 2.5" new drive uses only 1amp to start the motor.  (a vastly more modern drive) that has hope of working at USB3  Super speed mode. and gen x2 for sure)
As you can see USB has power limits.  (we always use a driver enclosure 12vdc 2amp power pack. and it never fails on any PC nor any HDD)
100mA is 0.1amps.  so 900mA is 0.9 amps. As you can see Type-C is best the C means charging up batteries rated (quickly) this also helps HDD big time.
Some lame PCs old share the USB Power feed, so  it is so very easy to overload these PCs with too many USB devices. (cheap PCs these are)
Wiki made chart, thank you wiki. (in order of hopeless to good)
The China made enclosures must work with 10,000 PC's made and 10,000 HDD made, and many will fail.
(if you don't use the power pack it came with or lost it count on failure.)
below is more proof.

 What to buy is easy: (DIY?)  Choices lots.
  1. Form factors, A full enclosure, 2.5" or 3.5? or even both , a docking station box and last just  cable adapter for just cloning or just erasing (DBAN) drives.
  2. A BOX that has USB port and cable  eSATA:
  3. An e-SATA port and cable.  SATA-III is best, for now and the future and for sure using any SSD.
  4. A 12vdc power pack rated at at least 2.0Amps or more.  (the startup current can be very high, 1 to 2amps or more as seen below.)
  5. Some Green drives, are only 1.2amps on the 12vdc bus,  3.5"  (about 1/2 power) starting up and getting to max RPM. (eg: WD10EZRX by Western Digital)
  6. Fan in the box is good for hdd that run hot, many do....for sure the fast ones. HDD some do 15,000 RPM and run hot.
  7. Costs? .
  8. Cute (nah) unless 13years old.
Here are some nice current  graphs (best of  best) showing how motor starts up,  the bigger the disk and faster it spins the more current it sucks (from the power pack or worse the gutless wonder USB ports on most PCs)
From Seagate white pages, that ramp down is the RPM coming up to speed. Those tiny spikes to the right are s the head servo moving or unlocking heads then moving to track 1. 
The heads fly and not until spec. RPM is reached at 5 second point in time. Do not let weak power put the drive in loop, from 0 seconds to 6 seconds, or  you may kill the drive dead (heard clicking over and over, bad bad bad)

As ygetting over this power hump is impossble with most PCs made for sure OLD PC's.
This newer modern 2.5: Seagate hdd  model , 2TB sizes, ST2000LM007 , run on 5vdc only and needs only 1amp to start the motor, and 0.1 amps idle, and 0.36amp  running average.
All things equal the 2.5" with 7200 rpm not 5400 rpm uses more startup current , one trick is buy 2 to 1 USB cable, that doubles USB power.  Seen below.
The larger the platter mass and high speed, the more current is uses to start up and run.
What your HDD needs nobody knows of of the 10,000 models made., learn to read its data sheet first.

Know that the USB  ports can go to sleep. (it's programmable by you in Windows CP + power + advanced) So turn it off I this feature gets in your way.
  This is my Desktop  USB port, It can not in anyway ,handle the startup current of my 2 TB  3.5" HDD. not possible.!!! (it's only 1/4th the power needed) But sure a SSD will sing like a bird here.
SSD drives do not use  12vdc, and the 5vdc power is vastly more small, but my page here, is HDD drives not SSD (SDD driver are cake walk easy, just plug it in and run it.
The word current in electronics means Amperes, of current flow of electrons, not "the most current (as in newest)"
The drive must see 5vdc and on 3.5" 12vdc at the correct voltages at all times, and for sure starting this motor and spinning up to 7200 RPM, If the voltage sags (means overloaded) then the motor can not start, up.\
This 500mA is not enough power (current) to start any drive made, only special ultra low current Green drives might start, ok>? Most modern 2.5:" drives need a full 1.0 amps current below is 1/2 that on tap.(fails)
See this motor run?, it uses lots of power ! (RPM and platter mass sets current needs)

This is one of  my drive cases, it also uses eSATA,  cables.
The Case comes new with  large power pack, USE IT AND WIN.  Infact this Case above runs on ANY PC made no issues at all. (ever) Because it is SELF POWERED properly.

Learning point 1:
Old PCs with old USB ports were never designed to charge batteries (CELL phones?) nor run current hogging HDD of any kind.
Take that as a fact. (some are only 100mA rated ports, and are HOPELESS here)

 HACK1: (this can work or not,  it is a marginal solution to this problem)
One can increase USB power by 2 times with this cable (or the like  that matches your case)  the left usb seen here may need to be the square kind , seen above. Type B is square above, type A fits PC port.
I know for a fact these cables can work, but for a fact it will on work on all HDD made, after all many HDD will not startup with  1 amp. of current. (a fact)

If each port is 500mA rated now you have 1amp to start up the motor. (USB3 would be 1.8amps)

My hard drive external failure list: (dead)  (bad drive, dead USB port, bad cables, wrong cables , wrong power pack)
  • Connect power cable, (both ends, HDD case and to 120vac wall powe jack.
  • Connect USB3 cable.(or USB 2) to the running PC.
  • PC goes dingdog, (means USB found new device)
  • You put your  ear to the HDD case side, and hear the motor start , it sounds like very tiny jet engine spooling up. The pitch heard rises like sliding your finger UP a  Guitar "E" string) (in 5 seconds time , power on)
  • The end sounds heard are  purring sound. (or tiny clicks as the heads jump and move tracks (cylinders they are called)
  • If the drive just clicks over and over loadly the HDD is bad or the power pack you just plugged in is Wrong or Dead. you forgot step 1 above. bullet 1.
First off it is only dead if missing in the Diskmanagment page ( run box,  type "diskmgmt.msc"  with no quotes)
 If it shows up there, then format it if want it to work the first time, then give it a drive letter. (waring Windows hates alien drives)
What I do is  DBAN all unknown used HDD first. (not mine, and not full of my data, but used HDD) Windows hates alien HDD formats.
  1. Expecting USB to power a HDD and not fail?, or lost your enclosure power pack 12vdc 2amps.(min.)
  2. As per above, the HDD sounds dead, silent  ear to case it is dead. (power good) The motor must make sounds like the MP3 link above shows.If not the HDD IS DEAD.
  3. Running old and  lame now,   XP the Virus magnet?  and the dead USB ports it is INFAMOUS FOR?
  4. The BOX (hdd enclosure) is turned off, or the power pack to it dead, or unplugged or the wrong one is used, 12vdc, 2Amps (min).
  5. Your USB port is dead or the driver for it is wrong ,missing or corrupted. Try all USB ports, USB3 is best (if present or add  a card with it)
  6. Forgot to format it? forgot to assign it a DRIVE LETTER? (if not explorer can't see it , only Diskmanagment  can.
  7. The drive was used, not by you, and is ALIEN formatted, then DBAN it , (full  erasure works).
  8. Take the drive and put in another PC direct to the motherboard and test it,? for sure run Linux demo mode live boot and test the Drive, using Linux. If dead now the drive is dead.

The External HDD is:  The drive maker seagate used Ocillicope and a nice current probe to do this, (means it's REAL)
Seagates current 3.5" "Desktop HDD series" (runs 2 to 2.5AMP startup in red below, on the  12vdc line.
This current is huge, and power on 2.5inch  is 5watts max but this one is 30watts,  6 times more.!!! 6 Time more power is needed to spin up this large HDD.

This is an Enterprise drive:
The green line is 5vdc turning on .(the step)
 The red line is 12vd startup current on the  ST6000NM0235 drive.
Again these are scope views with current probe on the motor power wire. 2.52 Amps to start up motor (in red)
I hate to tell you USB can not run this drive for POWER. Unless TYPE C or better. (charger current rated USP ports may work here, I have not tried it)

idle current is about 0.5A(12vdc bus) after the startup ends.
This Drive run great in my powered external case USB and eSTAT both work.

eSATA: (cable connected HDD) (some PC can even hot swap the cable setup correctly)
Some laptops do not have a 12vdc pin on the eSATA port,  making the motor dead, on any connected HDD. (unless you connect a 12vdc power pack to the drive.)

I use only 3.5" drives for  backup and are enterprise grade, and 2TB or larger. (I also have backup servers at home and on the www )
This drives have longer life span 10 years, is what they  unlike toy grade used, laptops 2.5" drives that have 1/2 the life span of good drives. (real SAS drives even better)
Mine site turned off, until I backup.

MY RECOMENDATIONS: (for Case/enclosure boxes)  I like dual sized drives and only with a full 2amp power pack or larger amps. USB3, works for cloning or backup duty)

 Gravity is cruel. (it is small and is portable and accidents do happen)
The dopping of any HDD to worst case concrete will kill it dead, from 4 feet high running.  The stopping distance is the cruel part of G-forces, if very short, time wise the disk is wrecked easy.
The heads fly running, if you cause them to hit the media, the game is over, for sure.
Turned off the HDD locks the heads and can take huge drops and hits. (head locks upward, or moves to a  head safe zone,)
Carpet is 30ms stop time.
Hardwood floor 10ms.
Concrete, well lets just say it goes off the scale on the left side, called the DOOM OF DEATH event.
My 3.5" HDD desktop in external case,  has a running (operating (write) spec of 40G max.  and 250G turned off (heads locked then) ST10000DM0004 .
Some Seagate 2.5" can take 400G/1000G (oper./parked) shock forces. (some cheaper laptop drives are not near this good ,like 250 opr. shock max.)
My HP Elitebook has a free fall sensor , and some HDD have inside the drive, a free fall sensor, (and locks the heads fast to safe zone) All I can say is learn to read your own HDD data sheet.
simple graphs make explaining easy,(physics 101) what matters is height and stopping time. (weight/mass is known)
Most drop distances are from waist height or desktop high, not when hang gliding or base jump suit heights. 3 foot to 4 foot max,.

version 4  8-8-2017  (rough draft, It is really just my journal , I share)