I have pulled out Seagate 2.5" 1 TB SATA HDD from my laptop.

The specs on the label say +5V DC 1.0A

So what am I wondering is how to interpret the current rating? There's no info on Wattage anywhere on the label.

Does it mean it needs at least 1A current or does it mean the max current drawn by this HDD under peak write shall not exceed 1A?

  • 1
    Ohm's Law: V[olts] x A[mps] = W[atts] [V*A=W ] (Standardized: Volts = E or P (Pressure) | Amps = I (Current, e.g. the amount of pressure) | Ohms = R (Resistance - e.g. akin to a valve)). Provided two of the values are known, the third can always be calculated. Ohm's Law applies to the relationship between Volts, Amps, and Ohms, normally used to calculate Watts; however, Joule's Law is used by those who work w/ electricity [electricians, electrical engineers] since it applies to power output (explanation)
    – JW0914
    Commented Jun 18 at 12:54

1 Answer 1


It means the maximum current drawn should not exceed 1 amp at 5 volts (ie 5 watts).

What is typical power consumption of 2.5" laptop external hard drive connected to USB socket? may be useful reading for you. (Most laptop hard drives are reasonably power efficient - ie cheap - 2.5" internal harddrives with some logic to convert SATA to USB added)

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    I had reason to look into this when adding USB drives to my Raspberry Pi 4. Typically a 2.5" USB powered drive seems to draw around 1.0A at power up, and when seeking, and much less at other times. Commented Jun 18 at 14:26
  • That's the one case where it matters - SBCs, especially the early RPis tended to run 'fine' on lower power USB PSUs until you started hanging off higher power
    – Journeyman Geek
    Commented Jun 19 at 0:05
  • @JourneymanGeek - I don't know about other boards, but with the Rpis, model 1 and 2 would run OK by themselves with an ordinary 5v phone charger, but 3 and later the official Raspberry Pi power supply (not 'charger' - chargers are not optimized for stable voltages) was very strongly recommended, to the extent that official Rpi forum posters about instability are always asked if they are using an official PSU and asked to use one and report back. Also they supply 1.2A max across all USB outlets (up to Rpi4, the 5 can do 1.6A with the right PSU) Commented Jun 19 at 22:01
  • I was thinking an OG Rpi actually but a lot of devices run off 'chargers' not 'Power supplies'
    – Journeyman Geek
    Commented Jun 20 at 0:17

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