I use an 8GB USB flashdrive (aka thumbdrive) for ReadyBoost on my desktop computer's Windows 7. Once in a while, I notice that Windows loses the drive, and so ReadyBoost no longer helps speed up the system.
(Sometimes, the reason is that Windows 7 has unhelpfully shut off the power to all USB hubs. See solution that I found to this problem here.)
But this time Windows reported just 200MB total disc space on the 8GB flashdrdive. I checked it on another Windows computer, and the same report. I formatted the flashdrive, but even then just 200MB were free. Did the drive lose most of its Flash RAM? It was too new and from a too reputable vendor, Imation.
I plugged the flashdrive into my Linux netbook, and it saw what Window did not: somehow, while sitting in the back of my desktop computer and just doing its ReadyBoost duties, it got partitioned -- meaning, it acted like it was two drives. The main partition was 200MB, the part that Windows saw; the secondary partition was not active, and so remained invisible to Windows.
But Linux saw them both. I used Linux to remove the second partition, and then reformatted it again with NTFS. The full 8GB returned for Windows to see.