4 yrs for any of those drives? time to retire them to archive purpose, extra backups.
Aim for 50% free / used
WD RED are for RAID and NAS and unlike green are 7200 rpm but at similar pricing as WD Green
Adding a SSD PCIe controller can let you move your system out of drive bay.
Using a drive in lower optical drive bay works too to add 5th
External drive for at least one backup set. Some externals are hot swap and therefore just put in drive for backup and then put it off line on shelf. 2 - drive bays $75
I agree that the warning from T.M. should have just remove old, but it always needs room even for temp files. I am waiting after 6.5 yrs for T.M. methodology to mature 😟
You have to remember the days of SCSI voodoo and all the things we had to learn about active termination, cables, what slot to use and the rest!
Cloning T.M. is not in the cards.
Start fresh with a new drive for TimeMachine backups. Period. Until you know or just forget it but that drive needs a full zero all - about 6 hrs on a WD Black today.
System drive: SSD $100
TimeMachine: WD RED $120 3TB
Date: WD Black 2TB
Sonnet Tempo PCIe SSD SATA: $150-300 (optional expansion and performance)
A nice clean OS install to get on a solid foundation, then use Setup Assistant ONLY if you trust where the data is coming from.
For every and each volume, CLONE it at least once and then update with CCC "as needed." Two clones is ideal as redundant safety precaution and so one set is always live and one is off line and safe.
Data Rescue 3 first. Disk Warrior 4.x+ second.
Carbon Copy Cloner on non-TimeMachine volumes has an Advanced Settings : Option - do a checksum on each and every file. Takes longer but insures you get a good copy.
So you may need to "recover" files once, then clone the recovered files a 2nd time just to be sure you have good data.
Disk Warrior can do a long thorough "scavenger" of the drive and bypass the directory and work with what it finds out on the drive and assemble a new directory.
From its Preview window you can see the volume's Before / After and you can copy manually (don't think you can clone but might) any files you need.
Disk Warrior in extreme cases has run for days. Successfully.
TechTool Pro 5 and later would work sometimes but it was able to CREATE and write new directory changes in low space situations.
That almost sounds like something you want someone who has done it all once or more and also buying programs for one-time use possibly - compared to if you owned in the past.
That is why backup sets are the best maintenance and avoid the cost and all of those software programs.
To know that your drive is 'healthy' one of the best bets I think is a known reliable program that runs in background and reports bad sectors and can map them out. SoftRAID 4 $140. Back 12 yrs ago the support and all was worth more than the program.
Windows. I use it, but for some they get along just fine with using it in a VM rather than dual boot and run Windows natively. And avoid a dedicated drive (though that is the best way if you use Windows in dual boot and enough to justify).
RED. RAID Enabled. Suitable for RAID5 and NAS. Firmware optimized for RAID and time limit error recovery feature. WD Black 4TB drives are $399. You could have 3-4 x RED (3TB) in a mirror for backup set using RAID5 or even mirror (RAID1) or RAID10 even. Drive box and other equipjment extra but nice LAN backup.
Some people do a lot of media streaming on their LAN and want something to store all their media library and access from.