When USB drives do begin to fail, they do so one cell at a time, not across the board, says Crandall. This is why an SLC-based drive might be worthwhile for a user storing, say, a virtual machine on a flash drive to restore their system after a disaster, according to Wilkison. If the drive began to fail, dropped bits might not be noticed in a photo or music track but could crash the system if they disappeared from a key part of an operating system.
There are features in the I/O controller that can boost performance in USB drives as well. One is the use of multiple channels to simultaneously move data to and from memory, says Brad Anderson, director of product marketing at USB flash drive vendor Lexar Media. Another, he says, is interleaving, which intermixes data flows to and from multiple flash memory chips within the drive to ensure the channel is used to its maximum potential.
As with many other design details, it's difficult for consumers to determine which I/O controller is used in a specific drive and which features it provides. While a consumer could try to find out which I/O controller is used in a specific drive and investigate how that controller works, most buyers will, according to Wilkison, have to "extrapolate from the speed" of the drive what type of controller is in it.
Right now, possibly the only way to know whether you've bought a drive that uses SLC memory is how much it costs -- the more expensive the drive, the more likely it is to have been built using SLC.
There is one way to judge the quality of a drive besides the price: the Windows ReadyBoost logo, which indicates that the flash drive can be used to supplement system RAM and thus speed the performance of Windows Vista-equipped PCs.
However, the minimum specifications for Windows ReadyBoost are just 2.5MB/sec. for random reads of 4KB of data, and 1.75MB/sec. for random writes of 512KB of data, which Crandall says are typical of lower-priced and lower-speed USB flash drives. Customers who want the maximum benefit from Windows ReadyBoost should opt for a USB drive marketed as a high-performance device and probably priced at the high end of the average for its capacity.
In the first half of 2009, Wilkison predicts, controller manufacturers will begin shipping drives with dual- and even four-channel controllers, which will increase speeds even for slower MLC memory by increasing the number of lanes through which data can be written to and from the memory cells. USB drives combining four-channel controllers with MLC memory will reach speeds of about 60MB/sec. for reads and 30MB/sec. for writes, coming closer to -- but not quite -- saturating the USB 2.0 interface.
For the average consumer for whom price is more important than speed or reliability, any reasonably priced USB drive should do. But if speed or longer life is critical, look for drives advertised as high performance, do your research online and expect to spend more. But even then, you can't be absolutely sure you're getting more speed for your money.