Backup Exec 2012 Job status Ready; No idle devices are available.

Your drive serial number has changed; at a minimum, you need to point the backup jobs to the replacement tape drive (which BE sees as a new device).

Did you replace the failed drive with the same type of drive (i.e., LTO-3 with LTO-3)?  If not, what was the old drive?  What is the new drive?

It doesn't sound like it should be necessary, but do an explicit inventory of tapes in the library.    

When you say "Everything installed fine..." -- did you install more than a tape drive?  If so, what else?  Did you re-install Backup Exec?

The backup jobs are pointing to the replacement drives.

The old drive was a HP Storageworks MSL2024 and the new drive is a HP MSL6000 Series.

I'm not quite sure what you mean do an explicit inventory?

Apologies for the "Everything installed fine" comment, it may have been slightly. The tape drive drivers installed fine.

I'm a bit confused -- an MSL XXXX is a tape library.  It will have one or more tape drives, and a number of tape cartridge slots.  The tape drive (or drives) that goes in an MSL library can be from the DLT/SDLT family, or from the LTO family.

Which leads to these questions:
1) Did you replace the whole library, or just the drive in the library?
2) What model number was the old drive? (i.e., SDLT-something or LTO-something; LTO may also be known as Ultrium XXXX.)
3) What is the model number of the new drive (or drive in the new library)?

The MSL6000 is an old, no longer sold library.  I worry that you had a newer generation LTO drive in your previous library, and that you've replaced it with an older drive, which will not be able to read your tapes written with a newer generation tape drive.

Okay. I'll try and explain the situation. We were using the HP Storageworks MSL2024 but it broke. I have just looked in BE and it has listed as HP Ultrium 4-SCSI. HP were telling me to replace the chassis of this but we have just recently removed extra equipment from an old datacentre we had and one of the pieces of equipment was the HP MSL6000 series so we thought rather than spending almost £1000 on a new chassis we would just use the other device.

I hope this answers all of your questions.

What you haven't told me is #3 -- what is the model of the tape drive in the MSL 6000 library?  There may be a sticker on the back.  If not, please take a look at the front bezel (if there is one) and see what it says.  Otherwise, power down the library and remove the tape drive, and look for identifying labels on the tape drive itself.

It would be nice to verify by inspection of the hardware (i.e. look at the thing itself, not at BE) which tape drive was in the MSL2024, if you've not disposed of it yet.

Drives are easily removable from both the MSL2024 and MSL 6000 -- a couple of thumbscrews does it.

I have removed the drive from the MSL 6000 and it is an Ultrium LTO 3.

Unfortunately, I don't have access to be able to inspect the MSL2024 as it is currently still in a very untidy rack.

I have just checked and all of the tapes are LTO-3 so I was previously using LTO-3 tapes in an LTO-4 drive. Also, there were some tapes already in the library of the MSL 6000.

Have you explicitly performed an inventory on the MSL 6000?  See http://www.symantec.com/connect/articles/inventory-and-catalog-what-they-do-and-when-use-them .  Please post a screen shot of the BE library view after the inventory.

In the end I purchased a new Chassis for the MSL2024