The difference between USB 1.1 and 2.0 is exactly what you mention: speed. If you’re putting a USB 1.1 device into a USB 2.0 port (or a USB 2.0 device into a USB 1.1 port) you’ll only achieve the slower transfer speed of 11 Mbps instead of the 480 Mbps you’ll potentially get if you plug a USB 2.0 device into a USB 2.0 port.

As for why some of the drives work with some machines and not with others, there are a few potential reasons. First, there are a few different USB controller chipsets, and while the spec is designed to be interoperable between manufacturers, sometimes there are drives that simply will not work with certain USB controllers. In cases like this you may want to try a different USB port on the other end of the machine, if one is available, since sometimes there are different controllers inside the same machine.

It’s also possible that there’s not enough power being delivered by the USB port on the machine where the USB drive isn’t working — some drives take more power than others, and if the computer is at the very edge of its power capabilities (lower power supply wattage, or just bad power delivery through the USB power pins) then more power-hungry USB drives can be problematic. You can often gain enough power to run them by plugging them into the computer through a powered USB hub.

Lastly, if you’re using these USB keys cross-platform (ie using them on Windows, Mac and/or Linux), you might want to check the file system for the USB drives — if they’re formatted specifically for Mac or Linux, they won’t be recognized by the Windows machine. You can fix this up by formatting them as FAT32 and reloading the data onto them.