USB storage without external power

SD Cards are inherently designed to be low power devices. USB devices are not necessarily designed with ultra-low power requirements in mind.

All USB devices have device attributes - that are interrogated by the host controller during the initial device-connection handshake. Among the attributes are the USB device type/class (e.g., printer, hub, smart card, audio, mass storage) and power requirements (in mA). This information ensures that the appropriate device-driver is loaded - and that the host interface can provide adequate power.

Unless independently powered, USB devices are powered by the USB bus. Some types of storage device require more power than a single (or multiple) USB port can provide - this being particularly problematic for small portable electronic devices that are themselves battery powered. As storage devices get larger, with faster interfaces, power requirements increase - and for this reason alone, many modern USB flash drives require more power than their older cousins.

Modern USB storage devices often declare a minimum power requirement of 500mA - this being greater than the Lightning port of an iPad or iPhone are designed to source. When used with an external power source (as supported by Apple’s Lightning to USB3 Camera Adapter), the iPad’s limited ability to source high power via its Lightning port is mitigated.

While you may be able to find USB flash storage devices that have lower power needs, you’ll likely encounter a substantial trade-off in both storage capacity and transfer speed - i.e., they will have low capacity and be very slow to transfer data.