Yes you can mix the two different drives but you must partition the library... 
And you must have a license for each partition....

-----Original Message-----
From: EMC NetWorker discussion [mailto:NETWORKER AT LISTSERV.TEMPLE DOT EDU] On 
Behalf Of Yaron Zabary
Sent: Thursday, November 08, 2012 11:37 AM
To: NETWORKER AT LISTSERV.TEMPLE DOT EDU
Subject: Re: [Networker] Is it possible to mix LTO-4 and LTO-5 drives ?

   As others have stated, mixed media can be an issue. You could add the
LTO-5 drives now, but work with LTO-4 media until you will be able to upgrade 
all drives from LTO-4 to LTO-5. Then, you should start replacing your media 
from LTO-4 to LTO-5.

On 11/06/2012 05:01 PM, xbosson wrote:
> Hello,
>
> My current Physical Library is a Quantum Scalar I500 with 4 LTO-4 drives. I 
> have frre space to install 2 additionnal drives but I 'm woindering if can 
> install LTO-5 drives. How the libray and networker will operate with 4 LTO-4 
> + 2 LTO-5 drives ?
>
> Thx !
>
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-- 

-- Yaron.