When you plug in any external drive in RecentX , the "Drive letter" assigned to it may differ. On one computer it may be E: and on another computer it may be F:.
So the data stored (all the files and folder paths) will be different.
Although you may run RecentX from USB flash drive, the data is however stored on the computer's fixed hard drive and not on the USB flash drive. So it is not self-contained.
This is the reason we store it in the PC itself (fixed hard drive).
You can still run RecentX from the USB flash drive. Let the data be stored on PC's hard drive. RecentX will work fine and you will be able to search your files and folders on your USB flash drive.
Just ensure that you add USB flash drive also in the list of drives in RecentX Preferences. After adding the drive as shown below, click on "Reindex" button and wait for 30 minutes or 1 hour so that all the files get indexed.