Did you manually modify the Samba conf file, or are you running defaults?
When the system booted, or when you mounted the NTFS volume, did you see any errors or warnings in the kernel logs? Can do a "dmesg | grep sdb" and see if there's anything funky or odd... If you have time just do a dmesg, and go through it looking for anomalies.
Test reading and writing from the linux box itself to rule out SMB and see if it's the drive.
I see your posted on FreeNAS forums... If you're running FreeNAS, is there an easy way you can tell if there's CPU wait on IO? If so it'll mean it's an issue on the drive, or Linux side (and not Samba).
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