OS X V10.7 Lion :: Cannot Copy Files To/from External Drive

I run an iMac 3.06 Ghz Intel Core 2 Duo with 10.7.3 and I'm having some serious problems with an external drive.  Some history first - I have a large iTunes library (~3Tb) of everytype of media which I stored on an external NAS (iomega StorCenter) with no problems - then this NAS started to report a drive error with imminent failure. I purchased a new DAS (Western Digital) drive and moved all my content from the NAS to the DAS and everything was ok.  Then I started to get errors through iTunes about cannot find the file and cannot read the files - and then I couldn't access anything on the drive.  At this time I thought this was a different problem because I was running preview Mountain Lion, or there was an actual fault with the disk.  After investigation, and coughing out for Data Rescue 3 I found that somehow the entire file structure had been corrupted and all my files had been orphaned.  So at this point I had one dying NAS drive and one DAS which wasn't usable and no space to recover any files to. 

So - next step was to purchase another external drive (and returned the WD) - this time I went for a G-Raid 4Tb (non-thunderbolt) drive and I've been in the process of recovering files from backups and iTunes.  I've had no problems until today when yet again I've started getting errors and now I have problems copying files to/from this G-Raid drive. 

Technical details - OS-X 10.7.3 - G-Raid 4Tb formatted as Mac OS Extended (Journaled) - currently showing 1,163 folders and 15,452 files. 

Symptoms - using finder drag and drop copy a file from the g-Raid to desktop results in 'The operation can't be completed because the item 'xxx' is in use. - 'The operation can't be completed because one or more required items can't be found (error code -43).' and the copy fails.

Using the cp command I get a 'Resource Busy' error.   However, this is not for all files - I've managed to copy all my books back to the desktop ok, when a couple came up with the above error I tried again and it copied successfully. 

I have run Disk Utility - here are the details :-

- 4TB G-Tech - VerifyVerifying partition map for “G-TECH”Checking prerequisitesChecking the partition listChecking for an EFI system partitionChecking the EFI system partition’s sizeChecking the EFI system partition’s file systemChecking all HFS data partition loader spacesChecking Core Storage Physical Volume partitionsThe partition map appears to be OK   Volume (Media) - VerifyVerifying volume “Media”Checking file systemPerforming live verification.Checking Journaled HFS Plus volume.Checking extents overflow file.Checking catalog file.Unused node is not erased (node = 24190)Checking multi-linked files.Checking catalog hierarchy.Checking extended attributes file.Unused node is not erased (node = 24784)Checking volume bitmap.Volume bitmap needs minor repair for under-allocationChecking volume information.The volume Media was found corrupt and needs to be repaired.Error: This disk needs to be repaired. Click Repair Disk.Verify and Repair volume “Media”Volume repair complete.Updating boot support partitions for the volume as required.Error: Couldn’t unmount disk.Verify and Repair volume “Media”Volume repair complete.Updating boot support partitions for the volume as required.Error: Couldn’t unmount disk.Verify and Repair volume “Media”Volume repair complete.Updating boot support partitions for the volume as required.Error: Couldn’t unmount disk.Verify and Repair volume “Media”Checking file systemChecking Journaled HFS Plus volume.Checking extents overflow file.Unused node is not erased (node = 94)Checking catalog file.Invalid sibling linkRebuilding catalog B-tree.The volume Media could not be repaired.Volume repair complete.Updating boot support partitions for the volume as required.Error: Disk Utility can’t repair this disk. Back up as many of your files as possible, reformat the disk, and restore your backed-up files. 

and it's just got worse - in getting the info for this post I no longer have access to the drive.   This is the second time this has now happened on two different directly connected drives (USB and Firewire) - which means the problem is elswhere.  I've a stack of errors in the system.log file 

May 10 17:52:16 Neil-Cattons-iMac com.apple.diskmanagementd[1057]: ** /dev/disk1s1
May 10 17:52:16 Neil-Cattons-iMac com.apple.diskmanagementd[1057]: ** Phase 1 - Preparing FAT
May 10 17:52:16 Neil-Cattons-iMac com.apple.diskmanagementd[1057]: ** Phase 2 - Checking Directories
May 10 17:52:16 Neil-Cattons-iMac com.apple.diskmanagementd[1057]: ** Phase 3 - Checking for Orphan Clusters

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Info:
iMac 21.5 - 3.06Ghz, Mac OS X (10.6.4)