1. If the pen drive is formatted with FAT filesystem, the largest file size allowed is 1 byte less than 4 GB. Attempts to create a larger file, whether by copying/moving or some other way, will fail with an error. The error message may vary depending on the software being used.
2. Due to the difference between a disk manufacturer's gigabyte, which is 1,000,000 bytes, and a file system gigabyte which is 1,073,741,824 bytes, an "8 GB" pen drive shows up to a computer as having 7.45 GB usable. (Don't know where the 7.56 figure comes from?)
3. Pen drives store data in flash memory cells that are grouped into pages, with the pages grouped together into blocks. Flash memory cells can only be directly written to when they are empty. If you are trying to jam a file into the free space that nearly fills it up, there may not be enough free blocks left, especially if the disk is fragmented. Also, as Patio notes, there is a certain amount of space reserved for file system overhead. One more thing, check that you have not got a bunch of "deleted" files in the Recycle Bin.
