USB 2.0 has a speed limit of 480 MBps. That doesn’t mean that it can transfer data that fast, only that that’s the theoretical limit. Most cheaper USB thumb drives and flash cards don’t bother to give you a speed rating on the packaging, which likely means that the transfer rate is quite low. A high performance flash device will likely give a rating such as 66X or 200X. These ratings are given in the same format as optical drives. 66X translates to 10 MBps. 200X translates to 30 MBps. The fastest you’re likely to find is 300X which would be 45 MBps. All of these are far, far below USB 2.0’s maximum transfer rate. But, if you know the speed rating of a USB flash drive, then you know that it will be at least that fast.

If you have an SD card that has a very high speed rating, but your card reader isn’t capable of reading that fast, then you won’t get the full speed. So, with SD cards, you need to check both the speed rating of the card and the speed rating of the card reader to be sure that you will be getting the full speed that it’s capable of producing.

Readyboost has some minimum requirements: 256 MB of capacity, less than 1 ms seek time, and the device must be capable of 2.5 MB/s read speeds for 4 KB random reads spread uniformly across the entire device, and 1.75 MB/s write speeds for 512 KB random writes spread uniformly across the device. If your memory doesn’t meet all these specifications, readyboost won’t work. Also, readyboost can only use a maximum of 4GB of RAM. If your device has more memory, it won’t be used.

So, the bottom line is that if you really want to do this to boost your computer’s performance, then buying high performance flash memory specifically for the task that you’ve checked out the speed rating on is advisable. Just using an old piece of flash memory you have lying around is not advisable.

Which one is faster?