Hi - we're operating the hard drive off an Apple Mac. I suspect the drive may be full but when we connect it to the computer now, we're getting an error message saying the disk is not readable by the computer. When we click 'initialise', and then click repair the error message is that the first aid failed because of an error on exit. It also says that the volume check failed and that there is an invalid b-tree node size. We are photographers and have a lot of large sized images on the drive. I'd appreciate any help you can give me. Thanks, Bert.

Hi Bert
Initialising is the worst thing you can do as that will erase the entire disk.
You need to get a copy of DiskWarrior from Alsoft. It is very good are fixing drives that report that error (disk utility very rarely fixes them - and even when it does, DiskWarrior will still find something wrong).
It sounds though as if the initialisation failed. That often means that there is a physical problem with the disk itself that software is not going to fix.
If that is the case then you need to get a copy of DataRescue from Prosoft. It is very good at recovering data off faulty drives. As long as the drive is spinning it has a good chance of getting everything back - however - it can take hours, and it also recovers every bit of crud you deleted in the past, sao you need an equivalent sized drive to put all the recovered files onto (you cannot put them onto the same drive that is being recovered) and then you need several hours to go though all the rescued files just to delete all the crud. It's a long job.
If the drive stops spinning you need the services of a profession data recovery service like ontrack - but that will cost youi.