On the Electrify, the external SD card is mounted at /mnt/sdcard-ext/ while the internal is at /mnt/sdcard/. The two are logically separate and cannot / should not be merged.
On the Mesmerize, the external SD card is mounted at /mnt/sdcard/external-sd/ as if it were a subfolder of the internal SD storage, which is also mounted at /mnt/sdcard/ like the Electrify. The two are just as separate as on the Electrify, however, and the subfolder isn't "real". If you connect your phone to your PC you will see two separate drives for them, and the "external-sd" folder on the internal storage will show up as empty — since it is!
I think what's happening here is that you want your data — downloads, possibly pictures and music, etc. -- from your Mesmerize's internal SD, and you thought it was all on the external SD. (I don't blame you, this system of having two areas both called SD is confusing.) What I would do is:
- Power off both phones
- Remove any external SD cards from both phones to avoid confusion
- Power on both phones
- Connect both phones from your PC and mount the internal storages on both to your PC
- Copy the content of the Mesmerise's internal storage over to the Electrify's internal storage
- Properly dismount both phones from the PC
- Power off both phones
- Re-insert your external SD card(s)
- Power on the Electrify and go happily on your way