Time Machine Now Impossibly Slow

Question

After the first long full back up nearly 2 years ago, my WD My Book Studio II has worked seamlessly completing Time Machine backups via FireWire in a few minutes every hour until about a month ago.
Several things occured around that time. Not sure in which order but I started getting 'back up disk full deleting older backups' warnings. It failed to complete a back up - I stopped it after a few days, yes days of trying. A few day before this I installed Sophos anti-vrus. 'Why after all these years did you do that,' I hear you say. I'm not sure. I suppose I was taken in by the malware splurge. Anyway I excluded Sophos from scanning Time Machine back ups and, in due course, turned Sophos completely off. However, I am still left with a back up that takes forever - it has yet to complete. Yes, I have stopped and re-started it a few times ( and the drive [actually took them out at one point to give them a hug!]). WD RAID Manager states that both 1TB drives in the case are healthy. I also noted that Spotlight was slowing the whole computer down by trying to index the back ups although when and why this started isn't clear. Anyway I have excluded TM back ups from Spotlight by placing it in the Privacy window in System Preferences. If I look at 'All Processes' in the Avtvity Monitor, not a lot is happening but the rate of back up is almost non-existent.
The TM widget states:
Starting standard backup
Backing up to: /Volumes/Time Machine Back Ups/Backups.backupdb
Waiting for index to be ready (100)
Waiting for index to be ready (100)
Node requires deep traversal:/Users/MervynG/Pictures/iPhoto Library reason:contains changes|must scan subdirs|fsevent|missed reservation|
No pre-backup thinning needed: 138.63 GB requested (including padding), 155.06 GB available
Bulk setting Spotlight attributes failed.
Bulk setting Spotlight attributes failed.
Copied 7.8 MB of 114.5 GB, 109 of 20815 items
I am suspecting that this may be a Sophos (virus checker) issue but, having turned it off, I'm left wondering how best to resolve the issue. Currently instead of a small back up it is trying to back up 123GB (of a 650GB hard drive) at a snail's pace. One other possibly related factor is that my iMac hard drive is nearly full (40GB to spare).
I have read Pondini's truly comprehensive and marvellous advice, in particular A6, and understand that for whatever reason TM may be undertaking a lengthy procedure. But in my case lengthy is measured in weeks and days rather than hours. Along the way trying to achieve this one new back up, TM has deleted nearly 18 months of other back ups apparently to make space.
I can't see any other threads describing quite this scenario so can anyone help?
Is (was) it Sophos or something else? Will it get to the end of this 'full' back up before Christmas and suddenly produce lost back ups having actually been doing an incremental back up (Pondini item D2)? Should I let it run? Should I uninstall Sophos? Should I re-format the WD drive and start again?
All wisdom and advice gratefully received.

Answer 1

Thank you both.
Sophos had an uninstall application in its folder that I used to remove the application itself. I assume that that was ok.
Have now re-started in Safe mode and then again normally.
The WD doesn't sound or seem different i.e. its hard drives are slowly and irregularly clicking away in a manner that suggests it is still not functioning as it once did. How do I know? Well I think it sounds different and it goes on and on and on ..... It never gets there and never stops until, that is, it fails (to complete). Also it seems to slow other computing activity.
Thus far each new attempt at a back up begins with a 'calculating changes', then proceeds to begin backing up but now at some point, usually a few hours in and with only a few MBs of the 123GB complete I get a spurious  'WD My Book Media is in  use and cannot be turned off' warning window.
The last two widget logs (see below) seem to suggest something was trying to eject the WD back up disk (it wasn't me; it just appeared). After this the tool bar icon has an exclamation mark and when I click on it it says 'Back up failed'?
Starting standard backup
Backing up to: /Volumes/Time Machine Back Ups/Backups.backupdb
Event store UUIDs don't match for volume: Macintosh HD
Indexer unavailable (200)
Node requires deep traversal:/Users/MervynG/Pictures/iPhoto Library reason:must scan subdirs|missed reservation|
Node requires deep traversal:/ reason:must scan subdirs|new event db|
No pre-backup thinning needed: 139.28 GB requested (including padding), 183.64 GB available
Bulk setting Spotlight attributes failed.
Bulk setting Spotlight attributes failed.
Bulk setting Spotlight attributes failed.
Bulk setting Spotlight attributes failed.
Stopping backupd to allow ejection of backup destination disk!
Starting standard backup
Backing up to: /Volumes/Time Machine Back Ups/Backups.backupdb
Event store UUIDs don't match for volume: Macintosh HD
Indexer unavailable (200)
Node requires deep traversal:/ reason:must scan subdirs|new event db|
Node requires deep traversal:/Users/MervynG/Pictures/iPhoto Library reason:contains changes|must scan subdirs|found in deep scan|missed reservation|
No pre-backup thinning needed: 139.28 GB requested (including padding), 183.62 GB available
Stopping backupd to allow ejection of backup destination disk!
I have carried out a number of 'repairs' since this issue first arose using disk first aid and each time there were no problems reported.
I do have a LaCie network drive that I occasionally use with CarbonCopyCloner as I read a while back that network drives couldn't work reliably with TM so I do appreciate that RAID 1 is no protection if the controller on the WD is misperforming.
So can I definitely say the drive hardware is faulty yet? The 'info' states that everything is healthy?
I will leave it running whilst I await your thoughts, however, I note that it is 615MB into the 123GB after only an hour so. This is odd (or perhaps normal depending on how you view it) as it has never got that far so quickly since this issue first arose.
Current on-going widget log reads:
Starting standard backup
Backing up to: /Volumes/Time Machine Back Ups/Backups.backupdb
Event store UUIDs don't match for volume: Macintosh HD
Indexer unavailable (200)
Node requires deep traversal:/ reason:must scan subdirs|new event db|
Node requires deep traversal:/Users/MervynG/Pictures/iPhoto Library reason:contains changes|must scan subdirs|found in deep scan|missed reservation|
No pre-backup thinning needed: 139.28 GB requested (including padding), 183.62 GB available
Bulk setting Spotlight attributes failed.
Bulk setting Spotlight attributes failed.
Copied 93.9 MB of 115.0 GB, 75 of 22265 items
Copied 530.6 MB of 115.0 GB, 89 of 22265 items
Thanks for the suggestions and advice thus far.