"are there any free apps people could recommend?"
There are no "free" data recovery apps that are user-friendly. They are ALL "pay for". They work like this:
1. You download the app for free
2. You run the app on the drive, it scans to see what it can find
3. If the app finds "recoverable data", it permits you to recover ONE file (and one only) as a "test"
4. If it looks like more files are recoverable, you then pay the fee, obtain a registration number, enter it, and the app "goes to work" on your problem drive.
Having written all of the above, there is STILL a reason why these programs might NOT be able to help you -- see below.
Just wondering -- do you have access to a PC that has Steve Gibson's "SpinRite" on it? I've never used it (I don't do Windows), but I've heard that it is the absolute BEST app there is, insofar as dealing with damaged drives is concerned.
Other thoughts:
Is the drive 4gb or 4 _terrabytes_?
How old is it?
Who made it?
A difficult problem when trying to access damaged USB drives that won't mount:
Even the best data recovery apps (Data Rescue 3 comes to mind) may not be of any use if the drive in question cannot be mounted on the desktop. I had this problem with a drive that had a damaged partition -- and the files on the damaged and un-mountable partition were those I wanted back.
But .... what to do next? No "drive repair" apps (notice that these are DIFFERENT THAN "data recovery" apps) could repair the drive to the point where the damaged partition was "mount-able".
My solution:
I RE-initialized the ENTIRE drive into a SINGLE partition (you are reading everything correctly). Yes, this destroyed all the existing partitions, and yes, it wiped out the drive's directory, as well.
BUT -- the data that was actually "out there" on the drive's sectors was left undamaged, because I chose NOT to "zero out" the drive, only to replace the damaged directories.
Once this was done, the drive was again "mountable" -- even though it showed on the desktop as an "empty" drive (no files at all) and ONE partition.
But -- because it was now mounted on the desktop, Data Rescue 3 could "see it". And DR3 was able to "do its magic" by by-passing the new directory and by going "right to the platters" to scavenge, and then re-construct, the data that it found out on the sectors of the drive.
The end result was I was able to get most of the files back. ALL previous folder hierarchies and most file names were lost (remember that file names and folders are constructs of the directory, which has been "wiped"), but the DATA itself was there.
As it happened, the files I was looking for were mp3's -- I dumped the whole batch (hundreds and hundreds) onto a drive that had a clean system (for this purpose) and into iTunes -- and iTunes was able to read the metadata that was still there, and in many cases re-assembled the mp3 files in their proper order.
The point of this whole story:
It -IS- possible to get the data back from a damaged USB drive that is no longer mountable.
BUT -- it takes work (MUCH work), time, the willingness to learn, and some MONEY.
You probably ain't gettin' there otherwise.
You will need
1. A SECOND external drive to serve as your scratch drive
2. Data Recovery software
3. Time