I use Time Machine to backup my iMac to a MacBook Pro which is running OS X server. Both are running 10.11.1; the Server version is 5.0.15. Time Machine had been running fine using a 4TB USB drive on the server. Yesterday I added a 6TB drive and switched the iMac to use it (again, over the network).
Now Time Machine fails after about 90 seconds and reports "The backup disk needs 743.84 GB for the backup but only 509.94 GB are available..." The Time Machine System Preference panel then shows "510 GB of 510 GB available," which is strange because before the failure it correctly indicated "6 TB of 6 TB available."
I have doubled checked the formatting of the 6TB drive (HFS+ Journaled, GUID Partition Map) and confirmed that the server also shows 6TB available. I have also reset Time Machine backups (http://pondini.org/TM/A4.html), rebooted both computers, shutdown and restarted the Time Machine service, and basically started everything I know of from scratch twice.
Any solutions or hints will be quite welcome!