MacOS – Disk Utility on Mavericks fails because “disk is too small”

I'm on mavericks and I am trying to delete a partition of an external hard drive. This is its schema

Partition1: 50GB  "MavericksBackup": Bootable backup
Partition2: 187GB "Home Data": Absolutely Empty
Partition3: 12GB  "Install OS X Mavericks": it contains only the bootable app to install Mavericks

Now I'm trying to Erase the Partition3 from Disk Utility and it fails with the following error:

The requested size change for the target disk or a related disk is too 
small. Please try a different disk or partition, or make a larger change.

What that even means?

I have tried to delete or resize also the Partiton2 and I get the same error.
I suspect the problem was created by Carbon Copy Cleaner: I've used it to copy everything from my internal HD to Partition1 to make it bootable. At the beginning of the process a warning message told me that in order to make Partition1 bootable I also had to create a recovery partition. I let it handle it.

Now, in fact I don't see the Recovery HD partition from disk utility but if I launch diskutil list I do get this (different) list of partition, including the Recovery one, not visible from Disk Utility:

/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  GUID_partition_scheme            *250.1 GB   disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2:                  Apple_HFS MvaricksBackup          49.9 GB    disk1s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk1s3
   4:                  Apple_HFS Home Data               187.3 GB   disk1s4
   5:                  Apple_HFS Install OS X Mavericks  11.6 GB    disk1s5

Not sure how to solve this? I would really avoid to reformat the whole disk…

EDIT: I started the disk utility in debug mode with this terminal command defaults write com.apple.DiskUtility DUDebugMenuEnabled 1 then Debug menu and Show Every Partition. This in fact shows the extra EFI and Recovery HD partition but that didn't help me solving the problem.