Pictures in iphoto not accessible on portable hard drive

We downloaded a lot of our iphoto pictures onto a portable hard drive (Toshiba). When trying to open them from the hard drive we get a message "OS X can't repair the disk "TOSHIBA EXT". You can still open or copy files on the disk but you can't save changes to files on the disk. Back up the disk and reformat as soon as you can." But we've not been able to either open or copy them.

We have a really old version of iphoto (v8.1.2) and old OS 10.9.5 so is this a factor? We have however been able to access photos previously, so I'm not sure why we've suddenly got this problem.

In Disk Utility log it says "This disk needs to be repaired. Click Repair Disk". But we a bit afraid of doing this in case it makes things worse.

Can anyone help?