They both mean the same. Actually a flash drive also called as a pen drive is an USB device ( Universal serial bus). This is called so because, the flash drives are attached with the computer at the...
The difference is the USB part. Some Flash Drives are USB, some are PS2 Port capable, the word Flash Drive is a rounded name for any Drive capable of information being put on it. The USB Flash Drive...
Technically speaking there is no set limit, as technology grows - so will the size of a USB Flash drive.
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To my knowledge the most memory you can have on a USB flash drive is 256 GB.
A USB Flash drive is considered a storage medium. Data can be both read, stored, and sent to / from a USB Flash drive, but it does not in itself input or output any data.