Unclear on the concepts, mate.
#1: USB is slower than real RAM and slower than hard dxrives. USB 2.0 Theoretical 480 Mbs vs. Theoretical 3200 - 8000+ Mbs for RAM. Plus USB imposes a CPU load which is undesirable. Of course, real life USB 2.0 throughput is far lower than the theoretical...
#2: Flash memory in keychain drives is slow - the fastest ones are in the 15 - 21 Mbs read time and considerably slower for writes.
#3: Because most affordable flash memory has to be erased in a block before being written to (that is, it does not support direct overwriting), Flash memory is REALLY slow for many, small writes... way slower than a fixed hard drive.
#4: Flash memory has a finite number of times that it can be written to before it dies. Not an issue for a camera card, which you'll write maybe once a day for a few years for a few thousand writes over its lifetime. But, worst case, as a scratch disk, the 'drive' will be written to thousands of times per hour. Sudden Flash Death predicted.
FireA -- the 'other way around -- using a hard drive as an extension of ram -- happens every minute of the day on your Mac, it's called Virtual Memory. OSX make extensive use of swap files on the hard drive, as does every modern operating system. But hard drives are way slower than RAM, which is why when you have too little RAM, and the machine is forced to use hard drive space for active memory, the machine slows down considerably.
Putting VM / swap space on a USB flash memory device would be a terrrrrible idea.
Using a different, faster kind of Flash memory from that which is used in consumer flash devices, it is potentially possible to improve performance, particularly hard drive performance - but then, hard drives already have 8 - 16 Mb of SDRAM on them for caching.
The problem with using Flash memory for RAM extension is that the Flash would have to be on the main memory buss, or something just as fast. If you put it on the SATA buss, or the PCI-e buss, then you're cutting the bandwidth down and increasing overhead. OK for hard drive caching, maybe, which is why we'll see hard drives coming in the future with big caches, but not for RAM. Then, if you are going to attach something to the Memory buss, why wouldn't you just install DDR2 memory, which is a LOT cheaper than any type of Flash memory that would be fast enough and survive repeated read/writes?
