Copying from Linux server to mapped NAS drive errs: cp: writing & cannot stat 'file': Input/output e

Hi.  These errors have just started occurring in a formerly very stable configuration.

From cronjob log:

cp: writing `/san_disk/Config_Files_Code_..._1211261904.tar.Zaa': Input/output error  
cp: cannot stat `/san_disk/hot_backup_..._1211261831.tar.Zaa': Input/output error  
...

I had to manually run the command and it still errored on one file out of 6

cp  /oracle_bups/${ORACLE_SID}/*_${today}* /san_disk 2>&1
...
cp: writing `/san_disk/hot_backup_..._1211261831.tar.Zac': Input/output error
...

I manually copied the erroring file again successfully.

(I know it says SAN, but it's NAS)

.

The fstab entry is

//.../linuxbackups$ /san_disk               smbfs   noauto,username=JMFSAN2\linux,password=...,workgroup=...        1 2