I recently bought a cheap 2Gb USB stick (a Buffalo) with the intention of speeding up both my media centre PC and my laptop, both of which run Vista. After plugging it in to the media centre PC I couldn't get it to work - Windows was convinced that it wasn't fast enough to be used for Readyboost. Ok, time to try the laptop - perhaps it was something to do with the on-board USB controller...
On the laptop I had more success. Initially, I had no trouble getting it to work - plugged it in and told it to enable Readyboost and off it went. I've been using it for several days without trouble. Now today I plugged it in (after having booted up my laptop this time, not before powering on), and I was told it wasn't fast enough. So what had changed? Well, the first time I tried it, I only had the stick connected to the laptop, and no other USB devices. Then, whenever I'd been using the laptop, I'd always plugged in all my USB devices when I powered on the machine. Today, I'd plugged in my external USB-powered hard drive first, then had to walk off to find the USB key before I could plug it in. By the time I got to the computer, everything was booted, and I was told it wouldn't work.
I have a Dell D610, which has two USB ports on the back and two on the side, so with the hard drive plugged in the back, I tried the side ports, thinking that they might use a separate controller on the motherboard, but that made no difference. So, I tried plan B, and unplugged the hard drive. Now, when I plugged in the USB stick it worked just fine. I then connected the hard drive and everything was back to normal. Of course, I retested the whole process and couldn't duplicate the fault (typical), but this course of action helped me when ReadyBoost stopped working, so I figured I should blog about it.
So, having had a good play with it on the laptop, I've just shut down my media centre box, plugged the USB stick in, and started up the machine again... hey presto - it works! I had to tell the system to re-test the device, but now it's working, and on that system (with only 1Gb RAM instead of 2Gb) I may even see some performance improvements.
I've heard people throwing around ideas about changing the file system type to get a USB stick to work (FAT, FAT32 or NTFS), but I must admit I'm not sure that would make much difference. Given that ReadyBoost data is stored in one single file on the drive means that accessing the file isn't going to be that big an overhead... though I guess it could be something to do with how big the cluster sizes are for each type of file system (getting rather low level and geeky here, but there's a good article on the Microsoft site about this.)