The backup disk needs 3.84 EB for the backup but only 5.44 TB are available.

A third-party network-attached storage device (NAS) or router is unsuitable for use with Time Machine, especially if it's your only backup. I know this isn't what you want to hear. I know that Time Machine accepts the device as a backup destination. I know that the manufacturer says the device will work with Time Machine, and I also know that it usually seems to work. Except when you try to restore, and find that you can't.

Apple has published a specification for network devices that work with Time Machine. No third-party vendor, as far as I know, meets that specification. They all use the incomplete, obsolete Netatalk implementation of Apple Filing Protocol.

Apple does not endorse any third-party network device for use with Time Machine. See this support article.

If you want network backup, use as the destination either an Apple Time Capsule or an external storage device connected to another Mac or to an 802.11ac AirPort base station. Only the 802.11ac base stations support Time Machine, not any older model.

Otherwise, don't use Time Machine at all. There are other ways to back up, though none of them is anywhere near as efficient or as well integrated with OS X. I don't have a specific recommendation.

If you're determined to keep using the device with Time Machine, your only recourse for any problems that result is to the manufacturer (which will blame Apple, or you, or anyone but itself.)

You don't have to throw away the NAS. It may have other uses. Just don't use it with Time Machine. If you're asking whether I can tell how to use it with Time Machine, I can't.

@Linc Davis (or anyone else who may have some insight),

I, too, am having a similar issue:

- OS X El Capitan w/ Server app, Time Machine Service enabled with a backup defined on on each of two different internal drives on a Mac Mini:

- 669.68 GB partition on Disk0

- 400 GB partition on Disk1

- Client is OS X El Capitan Mac Book Pro

- Backup size: 2 partitions of the one internal harddrive:

- OS Partition: 57.66 GB used // 74.11 total space

- Data Partition: 116.65 GB used // 424.8 total space

- Time Machine Preferences > Options > Estimated size of full backup: 174.11 GB

- Select Disk > regardless of which of the two partitions I select to use on the server, both error out with: "The backup disk needs 185.09 GB for the backup but only 147.58 GB are available. Select a larger backup disk or make the backup smaller by excluding files."

I've reformatted the partitions on the server; nothing is on the drives; no other clients back up to those partitions, so this would be an initial, full backup of the MBP to the server.

If I exclude the MBP OS Partition (57.66 GB), it will run successfully (leaving only the data partition with 116.65 GB in the backup), but I'd like to include both partitions on the MBP.

Any ideas on what is going wrong or how to fix the problem?

I have a similar issue.

1st mac mini el capitan w / server and 12TB external HDD. mini ethernet connected to airport extreme

2nd mac mini el capitan and another ext 12TB WD HD. mini wifi connected to same network.

from mini 1 i can select 12TB backup drive on mini 2 using time machine but after some time I get error:

'time machine couldn't complete the backup to '...server".

the backup disk needs 10.34TB for the backup but only 498.77 GB are available. Select a large backup disk or make the backup smaller by excluding files.

it seems that time machine on the server mini 1 is defaulting to internal 500 GB HDD.

any ideas?