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I have a 2014 Macbook Pro, and sometimes I plug in a USB Flash drive (the tiny ones) or microSDXC flash drive (microSDXC together with a USB adapter).

But even if I don't use the drive at all, just letting it idle, when it is plugged in for an hour or two, I touched it, and it could be 40-45°C (104-113°F).

If it is actively used, like if I copy files onto it once in a while for an hour, it could be up to 60-70°C (140-158°F). Is that normal? It is high temperature enough to cook an egg. Is the USB output from the Macbook Pro stronger than usual that make the USB drive hot?

(To make sure I don't have fake products I even ordered the USB flash drive and the microSDXC and USB adapter directly from SanDisk, so they should be authentic.)

P.S. I also tried and used a microSD adapter and used it on the Macbook Pro, and it was much cooler, so certainly we can use the microSD port, but what if a user wants to use the microSD port and the USB port at the same time for more storage? What if the user want a low profile USB flash drive as storage?

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  • What's the temperature of the MBP when you fry your eggs on the USB drive ? I wonder if (a) you can fry bacon on the MBP and make yourself a tasty breakfast and (more seriously) (b) if this is anything more than heat transfer by conduction ? I expect the USB drive makes metal-metal contacts with circuitry which itself has metal-metal contacts all the way back to the furnace, I mean CPU, and the temperatures you report do not seem unusual compared with other temperatures reported hereabouts.
    – High Performance Mark
    Commented Jun 4, 2020 at 6:26
  • I never said I fry my eggs on the USB drive. Maybe it is something that you do.
    – nonopolarity
    Commented Jun 4, 2020 at 7:46

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This is not an Apple issue because it’s the USB device itself (the microSD card) that’s getting hot, not the MacBook.

A device gets hot because electrons are flowing across the conductive surfaces causing friction. Those electrons are flowing because it’s in use.

Why does the USB-C Ethernet adapter get hot, even when unplugged from ethernet?

“In use” doesn’t mean when you are copying files to/from it. As long as it is being recognized by the system, it’s in use. To be candid, I am surprised to see temperature readings with such specifically and only a ~10% variance at idle and an even tighter variance at higher temperatures all measured by “touch.”

Is the USB output from the Macbook Pro stronger than usual that make the USB drive hot?

Unless the power is negotiated using the new Power Delivery specification, which it isn’t because an SD card is not a “power” device to be recharged like an iPhone, its receiving 5V at the current “requested9 (probably 100mA or less) because it’s attached to the USB bus.

One thing to remember is that a source (MacBook, charger, power supply, etc.) doesn’t push power. The device draws power.

Finally, just grabbing a random specification sheet (Transcend) for a SDXC card, the operating environment is -25C to 85C (-13F to 185F) , so you are well within its temperature ranges.

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  • but if you save that, SanDisk will say the same thing "this is not a microSDXC" issue. This is a computer issue, as the same card plug into other computers doesn't have that issue
    – nonopolarity
    Commented Jun 6, 2020 at 5:04
  • It’s a “variance” but still within the operating parameters. Again the computer doesn’t push anything to the card, the card draws the current. It’s a function of physics and electronics, not the Mac. Maybe there’s a compatibility issue, but it will be the accessory not compatible with the computer, not the other way around.
    – Allan
    Commented Jun 6, 2020 at 5:19
  • hm... is that so... in a way, it is like, a ball that is 20 feet high on a small cliff, if you roll it out, is the gravity pulling the ball down... or is it the ball "moving down the space"... you said the card "draws" the current or power... but if there is nothing to be drawn, such as air, then nothing happens? If you supply 110 volt to the sd card and the card is fried, you can't say that it is not due to the voltage, it is due to the card "drawing" the current and therefore it is the card's fault?
    – nonopolarity
    Commented Jun 6, 2020 at 7:39
  • But haven't said that, if the Macbook outputs 5V, and the card has lower than usual resistance, then it might be drawing more than usual power? As the power is V squared over R... so it might depend whether the V is provided too high, or whether R is too low
    – nonopolarity
    Commented Jun 6, 2020 at 7:41
  • So, if the card has lower than usual resistance, it still is must be the MacBook, right? It’s 3.3V that the SD card reader attached to the SD Bus uses to read cards. Your analogies are completely incorrect. If put 110 volt into something that doesn’t accept it, it will fry it. If I put a supply of 3.3V and capable of delivering 1000A, it’s not going to push all 1000A, it will only deliver what the load asks for. And if you overwhelm the resistance of the load with incorrect voltage, is a very different thing. You should know this, not twist words to fit your scenario.
    – Allan
    Commented Jun 6, 2020 at 15:46

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