USB is almost full, but I can't find any files on it

Hello,
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my USB started preventing me from changing anything on it, but I was able to copy some files from it to my pc. I ejected the USB and plugged it back in. After that I am unable to find any files on the drive. The PC shows that the drive is almost full (3GB of 30GB free).
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Until now I have tried:
1) in Control Panel -> Folder Options:
check 'show hidden files, folders and drives'
uncheck 'hide empty drives in the Computer folder'
uncheck 'hide extensions for known file types'
uncheck 'hide protected operating system files (recommended'
2) in Command Prompt (USB is on F:\):
typed in: "attrib -s -h -r f:\*.* /s /d"
- response: "File not found - F:\*.*"
typed in: "attrib -h -r -s /s /d g:\*.*"
- response: "File not found - F:\*.*"
typed in: "attrib -h -r -s /s /d g:\*.*"
3) using AutorunExterminator to disable "Autorun.ini" from:
http://ccm.net/download/download-11613-autorun-exterminator
4) scanning the USB with MalwareBytes from:
http://ccm.net/download/download-105-malwarebytes-anti-malware
5) scanning with ESET Nod32 antivirus but it found no threats, not even folders
6) recovering the files using Recuva from:
https://www.piriform.com/recuva
it found around 2,500 files, but almost 1,500 of those were corrupted
7) recovering the files using EaseUS Data Recovery Wizard from:
http://www.easeus.com/file-recovery/virus-file-recovery.html
it found around 35,000 files, but without names or addresses
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I'm kindly asking if anyone could share a solution for my problem or at least enlighten me if my USB problem is probably caused by a hardware malfunction.
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Thanks in advance!