OS X Mavericks :: Access Denied To Files Transferred Over Network

I have 2 Imacs, both running OS 10.9.3. The run on a simple office ethernet wired network. We have had no problems until yesterday when suddenly one of the Imacs (Imac A) disappeared from the network. Imac A could see Imac B but not the other way round. Imac A connected to Imac B  but was denied access to all the folders under the user directory on Imac B. The folders all have the no entry icon on them and if I try to open them I get the message "can't be opened because you don't have permission to see its contents" . I have checked that the sharing set up on both machines allows access read and write on items and we have been successfully sharing files for months. 

I have hooked up an old Mac Pro running 10.8.6. to the network. It can see both the Imacs but when I transferred files from that machine to Imac B I got the message "can't be opened because you don't have permission to see its contents". I've checked and the permissions on this folder are set for all to read and write. 

So far I have checked and repaired privileges using disk first aid. 

The Imacs are both 3.1ghz Quad core i5 running 10.9.3.