Flash memory cells can only accept a limited number of writes, hard drives have no limit. Flash drives minimize the issue by doing "wear levelling" - they distribute writes to different memory cells rather than always writing to the same place. Most flash drives can last for at least a few years under even moderately heavy write loads, but it's prudent to avoid using them in situations that are very write-intensive.
Usb sticks fail often, but not the CF cards I use intensively, so
- Take CF, not Usb sticks
- It's not related with Flash technology
Intel gives several years life expectancy at 1GB/hour write rate for its Flash SSD. Just try to write as much on a mechanical disk: it will fail much earlier.
You may try the DisableNtfsLastAccessUpdate in the registry. It reduces the number of writes to the disk, which also improves speed on older Flash disks. And of course, use Xp's improved write filter, and align the volumes.
Anyway, a 4GB drive is cheap now, so use it, backup, and observe.