Ok, the external drive is actually NTFS formatted. I deleted all files on it anyway, so no worries about loss of data. How can I reformat it to... HFS?
Ideally I would like to use the ext drive both for MAC and PC.
I appreciate your help.
I have used ExFAT as format to use the drive for both MAC and Windows.
The drive does work on my MAC now, but not on my Windows machine (...). I tried to select in Options both GUID and later Master Boot, but this didn't seem to help. My PC still tells me that the drive is not formatted. And it shows only NTFS as possible format (hence not possible to format it as ExFAT from my Windows). Could it be because my PC, to use an euphemism, is not the newest one on the market?
This is brilliant! Saved the day big time. Has the same exact problem. The fix is in the padlock on the bottom right hand corner. Clicked on it and I changed the Read only permission to Read and Write and then applied the changes to the whole drive using the gear icon and selecting "Apply to enclosed items ...". Then the whole drive was once again accessible.What format does the external drive use? It is listed under FORMAT.