The size of an external USB disk is 200MB rather than 2TB

What partitions are on the USB disk. Maybe there are multiple partitions and the first one is a small one.

The description field in "USB Storage Devices" says "ST2000DM 001-1CH164 [Partition 1]", so maybe there are other partitions, but I don't know how to access them. Only one partition is mounted by the NAS, it's called "USB_HDD_3_1", this "_1" is not on the device name which is "USB_HDD_3"

If you have a windows PC you can delete the existing paritions via the windows disk manager. Right-click on computer, then select manage. Find your drive, and select the volumes on it one at a time, and then delete them.

If this is a backup disk, then normally I'd suggest ntfs as the format, as it handles any ext3 file and can be read on windows and macs.

The disk is to backup the ultra2+ and is supposed to be used only by the NAS. NTFS is not an available option under 4.2.25. And "unfortunately" I only have Macs, and they can't format to NTFS. I will try to reformat the drive with the Mac with fat32 and try again on the ultra2+.

If you must leave a partition, make sure you delete all the partitions and create just the one. Since it is a 2TB disk you may wish to use the MBR partitioning scheme (see options when formatting).

fat32 is limited to 4GB file sizes, so it is not a good choice.

On the backup, I understand it is only written by the NAS. But if the NAS fails, I expect you will be quite annoyed if you can't read it on your macs.

You can delete partitions on the mac also, but since I don't own any I don't know the precise procedure.

Thanks to you both for your help.

I reformatted the disk on my Mac (there were indeed 2 partitions) as 1 fat32 partition (with MBR). The ultra2+ recognize the disk as a 2TB fat32 disk. I could then reformat it in ext4 and now it seems to run fine.

Stephen you're right: i'd be annoyed if I can't read this disk anymore (since a couple of month, I have every week new disappointment with Netgear NASs anyway). But I have access to other Netgear NASs, and it is possible to read ext format with a Mac (you need to install some extension), so I hope I will still be able to read it.

I'm surprised the ultra2+ don't reformat a disk as a single partition. Does it make any sense?

p