Hello, I recently bought a new iMac, the smallest size of the 27" screens. I have put lightroom and copied the catalog and everything worked fine when using my external drive (that was formatted for a PC). Finally I discovered that I wasn't able to delete or move the external drive files when using the iMac. I read that it was because it was formatted in NTSF. I have formatted the external to the journaled mac (OS? I can't remember exactly now). I did not choose the case sensitive choice. Anyways, before I cleared the external I put all the folders with photos on the new iMac desktop(temporarily). I then start to copy the photos back on to the external drive. It copies for a while, and then it will stop and say "can't copy this file, it is in use now(wording might not be exact, but that is the basic message). This stops the entire batch of folders that I was copying, even though it only mentions the one photo being in use. Then, if I try to copy anything to the external, I get the message (an unexpected error has occurred code 50). I have tried reformatting the external three times and staring over, each time it copies to near the same spot, and the problem starts again. If I reboot the computer, that has allowed me to copy a few photos (maybe 20-100) and then the problem starts again. When I go to the external and look at info, it says read and write a the bottom of that window. Sorry to make the post so long, maybe having all the info will help. Thanks so much to anyone who takes the time to answer!