cannot read 523976 bytes at offset 4696356680 (:1): ([21] The device is not ready. ).

I have a backup which is backing up some sql backup files and almost every day i get the following error

[Major] From: VBDA@server name "server name [/I]"  Time: 22/10/2014 00:40:15
[81:78]   GLOBALROOT\Device\HarddiskVolumeShadowCopy486\backup\backup\filename1.bak

 Cannot read 523976 bytes at offset 4696356680(:1): ([21] The device is not ready. ).

[Warning] From: VBDA@server name "server name [/I]"  Time: 22/10/2014 00:40:15

[81:77]   GLOBALROOT\Device\HarddiskVolumeShadowCopy486\backup\backup\filename2.bak
 Cannot open: ([2] The system cannot find the file specified. ) => not backed up.

[Warning] From: VBDA@server name "server name [/I]"  Time: 22/10/2014 00:40:15

[81:77]   GLOBALROOT\Device\HarddiskVolumeShadowCopy486\backup\backup\filename3.bak
 Cannot open: ([2] The system cannot find the file specified. ) => not backed up.

[Warning] From: VBDA@server name "server name [/I]"  Time: 22/10/2014 00:40:15
[81:77]   GLOBALROOT\Device\HarddiskVolumeShadowCopy486\backup\backup\filename4.bak
 Cannot open: ([2] The system cannot find the file specified. ) => not backed up.

it always first fails with the cannot read 523976 bytes and then the subsequent files fail with the system cannot find the file specified. The offest figure changes but the 53976 bytes stays the same.

any advice greatly appreciated

Thanks