This has worked just fine before, and I have been able to re-use them, just format them and install (burn with Etcher) a new iso-image. I have used them to boot and install Linux distros. I´m out of ideas and I thought that you might be able to help.
Note: If you're in a situation where you can install the hard drive into another Mac, then at Step 2 you would install it into the Mac instead of the external case and you'd also skip Steps 3 and 9.I have a question or problem that is a combination of Linux and Windows regarding creating usb bootable drives.
for Linux Mint) into the second Mac (i.e. Connect the external case to the Mac capable of booting from USB.Insert the HD into an appropriate external case.But for users comfortable with doing this, the process is broadly as follows: Obviously I can't cover all possible scenarios here, but this option is not so easy for an iMac (not even the 2007 model used by the OP). If you use an external case, then the process is similar to using Target Disk Mode, because a Mac started in Target Disk Mode is effectively an external hard drive. So, this option will be easier if you use an appropriate external case, and if the original Mac is not hard to work with. a HD from an iMac will not fit into a MacBook).
removing a hard drive from a Mac Pro is a lot easier than removing it from an iMac) and may also depend on the model of your other Mac (e.g. However, choosing this option may depend on the model of your first Mac (e.g. This involves removing the hard drive from the first Mac and inserting it into another Mac or an external case.
Linux Mint) onto the hard drive of the first Mac in Target Disk Mode (e.g. Select the bootable USB from the list and press Enter.Start the second Mac and immediately hold down the Option key until you see the startup disk manager.the one capable of booting from the USB). (Note: You may need to use an adaptor if the other Mac does not have native FireWire support - e.g. Use an appropriate FireWire cable to connect the iMac to another Mac capable of booting from USB.You'll know when it's finished by seeing an icon floating around the screen. Start the 2007 iMac in Target Disk Mode by holding down the T key during startup.The following is a general guide on what's involved: Both options requires access to another Mac that will boot from USB and the second one requires the user being comfortable with the process (in some cases the process would be a nightmare, in others fairly straightforward). Obviously the above options aren't for everyone. Remove the HD from the destination Mac and insert it into another Mac or external case.Start the destination Mac in Target Disk Mode.I see two ways to install an OS from a bootable USB on earlier Macs that didn't support booting from USB: So I offer this answer in the hope it works for you, but if not, it may help others. I haven't used this process to install Linux Mint, but I have used it to install other operating systems on earlier iMacs and MacBook Pros.