The data partitions on the ESXi USB drive are FAT partitions, so you are able to access them from within Windows or Linux. In Windows you may first need to assign drive letters to the partitions using the disk management tool.
Look for two partitions that are 250MB in size and for a file named state.tgz on them. Pick the most recent one of the two copies and copy it over to the newly installed USB drive. After the initial installation only one of the two 250MB partitions on this drive should contain files - replace the state.tgz file there with the copy from your old installation.
- Andreas