Frequent file system errors on USB drive

Hi,
I use a Samsung HD400LD USB drive for backups on Windows 7 Professional 
64 bit SP1. For some time now while running mirror backups I rather 
frequently have file system errors on that backup drive. Sometimes they 
can be repaired using chkdsk, sometimes chkdsk fails and I have to 
format the drive (did both, fast and full formatting). I am still using 
the same backup software (Backup4all), also tried another one (Personal 
Backup), same problem. It seems like the error occurs more on larger 
backups with very many files. But much larger backups with just one file 
(zipped system backup, Macrium Reflect) work without any problem. All 
backups have several GB of size.
There are no SMART errors, long self test passes without any problems. 
Memory test also without any errors.
What may cause this problem?

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