2008 R2 Backups running without error, but doing FULL backups twice a week...

Question

I have two Windows Server 2008 R2 systems which I am backing up with the native WSB utility.  Both are VMs under ESXi 5.5 on the same physical Dell R710 host.  Each has its own 2 TB USB connected device which was dedicated to and formated by and
for WSB.  Both complete their respective backups without error and have been up for months.
On system A, WSB reports there are 243 copies available and examining the drive shows there is 288 GB free of the 1.81 TB total.
On system B, WSB reports there are 11 copies available and examining the drive shows there is 1.33 TB free of the 1.81 TB total.
My question is, why doesn't system B leverage a similar amount of the drive?
Thank you for any ideas!
Clay
Newark IT Guy...

Answer 1

I may be way off base as I'm a real estate agent not an IT person and I've just started learning about WSB/wbadmin last week, but here are my thoughts:
1. Mandy's response is all about MaxShadowCopies which has little to do with the OPs issue (I think).  MaxShadowCopies is associated with "Client Accessible Shadow Copies" and related "Shadow Copies for Shared Folders." Useful for when
you accidentally delete or mess up a file in some way and you quickly and easily restore a previous version of that file via the file's properties window.  Going down the wrong path with this I think except for the small caveat that the absolute 512 shadow
copy maximum applies to WSB shadow copies.  Somebody correct me if I'm wrong.
2. system B may have hit the "Maximum Shadow Copy Storage Space" setting for that volume.  From the command line, use command "vssadmin list shadowstorage" and compare the "used shadow copy storage space" to the Maximum. 
If it is close, or if an additional backup would cause it to exceed the limit, the system will delete backups from shadow copy until there is room for the new one (again, "I think" this is so).  See the vssadmin resize shadowstorage" command
options for adjusting the size.
3. I've read a few examples where the free space shown via a drive's properties was wrong for various reasons so maybe this is your circumstance.  So, system B may have had much less available space than you think, and because of limited space, The new
full backup takes priority and WSB is automatically deleting older shadow copy snapshots in order to make room for the new backup.  Run chkdsk on that volume and see what the "available" space is.
Also, If it turns out you do have a space issue, Do you know for sure that your backups are succeeding?  If the low number of backups is due to limited space, it could also cause the new backups to fail due to not enough space.
Caveat on all of the above: I am quite uncertain about all of this, so hopefully someone will chime in and corroborate or correct my comments.  
As a parting thought, I'll have to say that after reading the forum for a few hours trying to learn about WSB/wbadmin it is disappointing to see, in thread after thread, so many canned responses linking the same reference material without any attempt to actually
answer the OPs questions or deepen understanding of the subject.  Is this forum managed by paid employees?  If so, it feels like many simply locate a document that "might" be related, then they copy and paste a link along with a few statements
directly from said document.  No added insights of their own, no furthering or revealing of nuances about the subject at all.  Mandy's post above is a prime example.   Just an observation from a new visitor to this forum.
PS - I did notice that "Sriram [MSFT]" posted some very very helpful comments on the WSB & VSS subjects, just wish there were more like his.