Most USB 2.0 flash drives look the same, but that doesn't mean they perform the same.

Differences in the type of memory and, to a lesser extent, the type of I/O controllers used by USB drives can make one device perform two or three times faster and last (theoretically, at least) 10 times longer than another, even if both sport the USB 2.0 logo.

Unfortunately for the average user, there are no accepted industry standards or certifications to judge what's inside a USB 2.0 flash drive on a store shelf. Without checking reviews or running benchmarks, the only rule of thumb is that the more expensive drives (and those which post performance numbers on the packaging) will tend to be the fastest and, perhaps, last the longest.

The casual user may not ever notice. "For the average user, moving a few files around, or even 20-50MB of data, a slower drive is probably sufficient," says Cameron Crandall, a technology manager at memory vendor Kingston Technology And while the memory in less expensive drives has a shorter life span than that in more expensive drives, even that is enough to last the lifetime of the drive for an average user.

However, these differences can matter if you're storing large amounts of data, using the drives to store critical information or are using a USB drive to supplement system memory using the ReadyBoost feature in Windows Vista.

Key performance factors

The USB 2.0 standard supports a maximum throughput of 60MB/sec., although "nobody's pushing that limit" with the flash memory used in current USB drives, says Pat Wilkison, vice president of marketing and business development at STEC, a manufacturer of memory and storage products.

The single biggest factor in USB drive performance is whether it contains one of two types of memory: SLC (single-level cell) or MLC (multilevel cell). SLC stores one bit, and MLC stores two bits of data in each memory cell. SLC is twice as fast as MLC, says Wilkison, with maximum read speeds of about 14 MB/sec. and write speeds of about 10-12MB/sec. Not surprisingly, almost all current USB flash drives are built using MLC memory, since SLC costs about twice as much as MLC.

Users would see the greatest performance difference between SLC and MLC if they were performing many operations involving small files, rather than relatively few read/write operations on larger files, says John Whaley, principal engineer at MokaFive, whose virtualization software makes it possible for virtual machines to be stored on USB flash drives.

SLC memory also lasts about 10 times as long as MLC, says Crandall, which means one cell in an SLC-based USB drive should last for about 100,000 cycles of writing and erasing data before it fails. However, this difference won't be of much concern to many users.

"If you save a file out to your USB drive and use 100 bytes, you're probably not going to write to those same 100 bytes again for a long time, unless for some reason you decide to delete that file or change it," says Crandall. As a result, "the usable lifetime of the device will extend long beyond" the customary five-year warranty, says Mike Sager, vice president of public relations at Kingston.