problems with seagate external hdd

I have a Seagate 3TB external HDD attached to my iMac via a Firewire 800 cable. I have the latest software running (Yosemite 10.10.3) but the Mac no longer shows the HDD in finder, I can see it in disk utility but when i come to verify or repair the disk i get this message

" Error: Partition map check failed because no slices were found" I also get this message:

Verifying volume “Seagate External Drive”Verifying file system.Checking Journaled HFS Plus volume.Invalid B-tree node sizeThe volume Seagate External Drive could not be verified completely.File system check exit code is 8.Error: This disk needs to be repaired. Click Repair Disk.

The drive is nearly full and I really don't want to lose any files, can anyone help please..