samtools sort bam file error: generate non-existent file

[mediator]

Hi,
The following is the shell script I wrote to use samtools for sorting bam files. The bam files are from pair end Tophat alignment.

#!/bin/sh
samtools sort /data/N1.bam /data/N1_sorted
.
.
.
.

The script seemed run OK. But when I do ls -l under the output directory, all the sorted file has a ? inside its file name, like N1_sorted?.bam. Those sorted files are there, except I could not download nor edit them. Any suggestions? Thanks.